JavaScript - Lost Calculating Vat Using Javascript

<form id="calculation" action="#" method="post">

100 <input type="checkbox" name="check1" value="100" onClick='total_cost()'/>

120 <input type="checkbox" name="check2" value="200" onClick='total_cost()'/>

Total cost<input type="text" name="total" id="total" readonly="readonly" />

// JavaScript Document

var tblRows = 20;
var tblColumns = 10;
var tblArray = new Array(tblRows);
var p = window.parent;

// Initial page event handler
function initPage() 
{
    p.refAssignCellFromTextbox = assignCellFromTextbox;

    for (var i = 0; i < tblArray.length; i++) 
	{
        tblArray[i] = new Array(tblColumns);
        for (var j = 0; j < tblArray[i].length; j++)
            tblArray[i][j] = "0";
    }

    recalculate();
}

// Recalculate cell values
function recalculate() 
{
    for (var i = 0; i < tblRows; i++) 
	{
        for (var j = 0; j < tblColumns; j++) 
		{
            if (tblArray[i][j].indexOf("=SUM") != -1) 
			{
                calculateCell(i, j);            
            }
        }
    }
}

// Click cell event handler
function clickCell(cellRef) 
{
    thisRef = cellRef;
	
    var cellID = cellRef.id;

    var row = parseFloat(cellID.substr(0, 3)) - 1;
    var column = parseFloat(cellID.substr(3, 2)) - 1;

    var cellValue = tblArray[row][column];
    var tokenArray = getFormula(cellValue);

    if (tokenArray != null)
        p.assignTextboxFromArray(cellValue);
    else
        p.assignTextboxFromCell(cellRef);
}

// Update cell from textbox
function assignCellFromTextbox(tblValue) 
{
    if (thisRef != undefined) 
	{
        if (tblValue == "") 
		{
            tblValue = "0";
        }
        var tokenArray = getFormula(tblValue);

        if (tokenArray != null) 
		{
            assignArray(thisRef.id, tblValue.toUpperCase());
        }
        else 
		{
            if (!isFloat(tblValue)) 
			{
                    parseValue = tblValue.replace(/[^0-9]/g, '');
                    tblValue = parseValue;
            }
            assignArray(thisRef.id, tblValue);
            if (tblValue == "0")
                thisRef.innerText = "";
            else
                thisRef.innerText = tblValue;
        }
		
        recalculate();
    }
}

// Determines if a user entered a formula
function getFormula(tblValue) 
{
    var pattern = /[:|\(|\)]/;
    var ar = tblValue.split(pattern);
    var sum = ar[0].toUpperCase();

    if (ar.length < 3)
        return null;
    else if (sum != "=SUM") 
	{
        return null;
    }
    else 
	{
        return ar;
    }
}

function assignArray(cellID, tblValue) 
{
    var row = parseFloat(cellID.substr(0, 3));
    var column = parseFloat(cellID.substr(3, 2));

    tblArray[row - 1][column - 1] = tblValue;
}

function calculateCell(row, column)
{
    var tokenArray = getFormula(tblArray[row][column]);

    if (tokenArray != null) 
	{
        var fromColumn = tokenArray[1].substr(0, 1);
        var fromRow = tokenArray[1].substr(1, tokenArray[1].length - 1);

        var toColumn = tokenArray[2].substr(0, 1);
        var toRow = tokenArray[2].substr(1, tokenArray[2].length - 1);

        var fromRowIndex = parseFloat(fromRow) - 1;
        var fromColIndex = fromColumn.charCodeAt(0) - 65;

        var toRowIndex = parseFloat(toRow) - 1;
        var toColIndex = toColumn.charCodeAt(0) - 65;

        var sumTotal = 0;

        for (var i = fromRowIndex; i <= toRowIndex; i++) 
		{
            for (var j = fromColIndex; j <= toColIndex; j++) 
			{
                if (isFloat(tblArray[i][j]))
                    sumTotal += parseFloat(tblArray[i][j]);
            }
        }

        var cellID = fillField((row + 1).toString(), 3) + fillField((column + 1).toString(), 2);
        document.getElementById(cellID).innerText = sumTotal;
    }
}

function isFloat(s) 
{
    var ch = "";
    var justFloat = "0123456789.";

    for (var i = 0; i < s.length; i++) 
	{
        ch = s.substr(i, 1);
        if (justFloat.indexOf(ch) == -1)
            return false;
    }
    return true;
}

function fillField(s,n)
{
    var zeros = "0000000000";
    return zeros.substring(0, n - s.length) + s;
}

// JavaScript Document

var refAssignCellFromTextbox;
var nCharsAllowed = "ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890.=(): ";


// Event Handler
function initPage() 
{
    document.getElementById("TableFrame").style.top = "50px";
    document.getElementById("TableFrame").src = "tablepage.html";
}

// Updates text field when a user clicks on a particular field from with the spreadsheet
function assignTextboxFromCell(cValue) 
{
    document.getElementById("fieldEntry").value = cValue.innerText;
    document.getElementById("fieldEntry").focus();
}

// Assists tablepage.js with updating a value from a 2D array
function assignTextboxFromArray(aValue) 
{
    document.getElementById("fieldEntry").value = aValue;
    document.getElementById("fieldEntry").focus();
}

// Used when a user presses a key to update text field
function filterText() 
{
    if (window.event.keyCode != 13) 
	{
        if (!nCharOK(window.event.keyCode))
            window.event.keyCode = null;
    }
    else 
	{
        window.event.keyCode = null;
        refAssignCellFromTextbox(document.getElementById("fieldEntry").value);
    }
}

// Checks to see if character entered falls within our preset
function nCharOK(c) 
{
    var ch = (String.fromCharCode(c));
    ch = ch.toUpperCase();

    if (nCharsAllowed.indexOf(ch) != -1)
        return true;
    else
        return false;
}

// Clears all data from the screen
function clearPage() 
{
    initPage();
    document.getElementById("fieldEntry").value = "";
}

// Add a row to the table
function addRow(frmTable) 
{
	var table = document.getElementById(frmTable);
	var rowCount = table.rows.length;
	var row = table.insertRow(rowCount);
	var cell1 = row.insertCell(0);
	// cell2.innerHTML = rowCount + 1;
	// Unless this is going to be completely dynamic
	// the rest of the table code would have had to be 
	// entered in statically. Not happening.
}

// Delete a row from the table
function deleteRow(frmTable) 
{
	try 
	{
		var table = document.getElementById(frmTable);
		var rowCount = table.rows.length;

		for(var i=0; i<rowCount; i++) 
		{
			var row = table.rows[i];
			var chkbox = row.cells[0].childNodes[0];
			if(null != chkbox && true == chkbox.checked) 
			{
				table.deleteRow(i);
				rowCount--;
				i--;
			}
		}
	}
	catch(e) 
	{
		alert(e);
	}
}

var dateofBirth="30/10/1982";
var createdDate="01/12/2010";
var sd = dateofBirth.split('/');
var dob = new Date(sd[2],sd[1],sd[0]);
sd = createdDate.split('/');
var submit = new Date(sd[2],sd[1],sd[0]);
var age=submit-dob;
var ageYears=Math.floor((((age/3600000)/24)+1)/365.25); 
if(ageYears>75) document.write("true") else document.write("false") ;

function countdown(){
	
	var bigday	= new Date(2009,10,29,14,30,0,0);
	var today	= new Date();
	
	var difference = bigday - today;
	
	var remaining = Math.floor(difference/1000);	// want seconds, not milliseconds
	
	var days = Math.floor(remaining/86400);
	remaining = remaining % 86400;
	
	var hours = Math.floor(remaining/3600);
	remaining = remaining % 3600;
	
	var minutes = Math.floor(remaining/60);
	remaining = remaining % 60;
	
	seconds = Math.floor(remaining);
	var out = days + " days, " + hours + " hours, " + minutes + " minutes and " + seconds + " seconds left...";
	
	$('#countdown').text(out);
	
	setTimeout(countdown, 1000);
}

<!-- Row 3, Col 3 purchase boxes -->
<td colspan="1" height="120" align="left">
<input style="margin-left: 60px" type="text" name="bed_359" size="3" maxlength="3" onchange="calculateValue(this.form)" /> R359</td></tr>

<!-- Row 10, Col 2 Order Value Box-->
<td colspan="1" align="left"><input style="margin-left: 60px" type="text" name="total" size="10" 
onfocus="this.form.elements[0].focus()" />
</td></tr>

// Function to calculate order value	
function calculateValue(orders) {
	var orderValue = 0;
	var value = 0;
	var itemPrice = 0;
	var itemQuantity = 0;
	
	// Run through all the product fields
	for(var i = 0; i < orders.elements.length; ++i) {
	
	// Get the current field
	formField = orders.elements[i];
	
	// Get the fields name
	formName = formField.name;
	
	// Items price extracted from name
	itemPrice = parseFloat(formName.substring(formName.lastIndexOf("0") + 1));
	
	// Get the Quantity
	itemQuantity = parseInt(formField.value);
	
	// Update the OrderValue
	if(itemQuantity >= 0) {
	value = itemQuantity * itemPrice;
	orderValue += value;
	}
	}
	
	// Display the total
    orders.total.value = orderValue;
}

function numberAdd1 () {
addValue = 1;
return.addValue;
}
function numberAdd2 () {
addValue = 2;
return.addValue;
}
function numberAdd3 () {
addValue = 3;
return.addValue;
}