Hi there and thanks in advance for any assistance.
I am writing a form and doing a bunch of calculations one of which is a percentage based on the value selected. I am returning a correct result for two of the values but for the other two I am not.
Clearly I am missing something and maybe not testing for it but as I am clueless I can't think of what.
The 'discount' value is what I am looking for. It should return as 0 for values of 1 but it is doing the math as though it is multiplying by 1. And again for the value of 6 it should be 10% but is taking the value as 4.6 or something.

Code is below I've included all of it (sorry) for better ease of understanding the math that is happening:
HTML Code:
<form id="form1" action="" method="get">
<fieldset>
	<p>
	<label for="Type">Type of Storage:</label>
	<select id="Type" name="type">
		<option name="Heated Storage" value="60">Heated - $60/month</option>
		<option name="Unheated Storage" value="50">Unheated - $50/month</option>
		<option name="Offsite Storage" value="40">Offsite - $40/month</option>
	</select>
	</p>
	<p>
	<label for="Length">How long you need storage:</label>					
	<select id="Length" name="length">
		<option name="Don't know" value="1">Unsure (month to month)</option>
		<option name="One month" value="1">One month </option>
		<option name="3 months" value="3">3 months (5% pre-payment discount)</option>
		<option name="6 months" value="6">6 months (10% pre-payment discount)</option>
		<option name="one year" value="12">One year (15% pre-payment discount)</option>
	</select>
	</p>
	<p>
	<label for="Gobox">Number of Goboxes:</label>					
		<select id="Gobox" name="gobox">
			<option value="1">1 - $70 delivery fee</option>
			<option value="2">2 - $100 delivery fee</option>
			<option value="3">3 - $130 delivery fee</option>
			<option value="4">4 - $160 delivery fee</option>
			<option value="5">5 - $190 delivery fee</option>
			<option value="6">6 - $220 delivery fee</option>
			<option value="7">7 - $250 delivery fee</option>			
			<option value="8">8 - $280 delivery fee</option>
			<option value="9">9 - $310 delivery fee</option>
			<option value="10">10 - $340 delivery fee</option>
		</select>
	</p>
	<p>
		<label for="delivery">Delivery:</label>
		<input type="text" id="delivery" size="10" maxlength="10" readonly="readonly" value="0" />
	<br />
		<label for="storageCosts">Storage Costs:</label>
		<input type="text" id="storageFees" size="10" maxlength="10" readonly="readonly" value="0" />
	<br />
	<label for="discount">Discount:</label>
		<input type="text" id="discount" size="10" maxlength="10" readonly="readonly" value="0" />
	<br />
		<label for="taxes">Taxes:</label>
		<input type="text" id="roundedTaxes" size="10" maxlength="10" readonly="readonly" value="0" />
	<br />
		<label for="total">Final Total:</label>
		<input type="text" id="finalTotal" size="10" maxlength="10" readonly="readonly" value="0" />
	</p>

	<p align="right" ><input type="button" id="submit" value="Get an Esitmate" onclick="processForm()" />&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</p>
</fieldset>
</form>


<script type="text/javascript">

function roundNumber(num, dec) {
	var result = Math.round(num*Math.pow(100,dec))/Math.pow(100,dec);
	return result;
}

function processForm() {
	//get form variables
	var theType = form1.type.options[form1.type.selectedIndex].value;
	var theLength = form1.length.options[form1.length.selectedIndex].value;
	var theGoBox = form1.gobox.options[form1.gobox.selectedIndex].value;
			 
	//set tax rate
	var theTaxes = 0.07;

	//set shipping rate
	var theShipping = 30.00;

	//variable for order total
	var orderTotal = 0.00;

	//the display box
	var deliveryBox = document.getElementById( "delivery" );
	var storageBox = document.getElementById( "storageFees" );
	var discountBox = document.getElementById( "discount" );
	var taxesBox = document.getElementById( "roundedTaxes" );
	var totalBox = document.getElementById( "finalTotal" );

	//Calculate Delivery
	delivery = (theGoBox * theShipping) + 40;	

		//Calculate Storage Costs
	storageFees = ((theType * theGoBox)* theLength);
	
	//Calculate prepayment Discount
	if (theLength = 3) {
		//multiply storage costs by 5% 
		discount = storageFees *(5/100);
		}
		else if (theLength = 6) {
		//multiply storage costs by 10% 
		discount = storageFees *(10/100);
		}
		else if (theLength = 12) {
		//multiply storage costs by 15% 
		discount = storageFees *(15/100);
		}
		else {
			discount = 0;
		}

	//Calculate Taxes
	taxes = (storageFees + delivery) * theTaxes;
	var roundedTaxes = roundNumber(taxes,2);
		
	//figure final total
	finalTotal = delivery + storageFees + roundedTaxes;

	//output to text boxes
	deliveryBox.value = "$" + delivery;
	storageBox.value = "$" + storageFees;
	discountBox.value = "$" + discount;
	taxesBox.value = "$" + roundedTaxes;
	totalBox.value = "$" + finalTotal;

}

	</script>