Round value using selection of drop down menu / radio button

Round value using selection of drop down menu / radio button

This consists of two problems. The first problem is rounding numbers to a
certain decimal point. The second problem is connecting the drop down menu
to the right decimal.
First my code:
<style>
span{font-style:italic;}
span{color:green;}
</style>
<script>
function calcul(){
var sales = parseFloat(document.getElementById("sales").value);
var OpExp = parseFloat(document.getElementById("OpExp").value);
var TaxAll = parseFloat(document.getElementById("TaxAll").value);
var Depre = parseFloat(document.getElementById("Depre").value);
var Divid = parseFloat(document.getElementById("Divid").value);
var TaxR = parseFloat(document.getElementById("TaxR").value);
//GP = Gross Profit
var GP = sales - OpExp;
//TaxInc = Taxable Income
var TaxInc = GP + TaxAll;
//NetInc = Net Income
var NetInc = TaxInc - ((TaxR / 100) * TaxInc);
document.getElementById("NetIncome").innerHTML=
TaxInc - ((TaxR / 100) * TaxInc);
//AtRE = Addition to Retained Earnings
document.getElementById("AtRE").innerHTML = NetInc - Divid;
}
</script>
<form action="" id="nothing">
In 2007 the British building firm Balfour Betty plc had sales of
<input type="text" id="sales" maxlength="6" size="6">
million, total operating expenses of
<input type="text" id="OpExp" maxlength="6" size="6">
million, a tax allowance (rebate) of
<input type="text" id="TaxAll" maxlength="6" size="6">
million because of past losses, and
<input type="text" id="Depre" maxlength="6" size="6">
depreciation. <strong>What is the net income of the
firm?</strong><br />
<br />
Balfour Betty plc paid out <input type="text" id="Divid" maxlength="6"
size="6"> million
in cash dividends. <strong>What is the addition to retained
earnings?</strong><br />
<br />
The tax rate is <input type="text" id="TaxR" maxlength="6" size="6">
%.<br />
<br />
<input type="button" value="Calculate" id="but" onclick="calcul()" /><br />
<br />
</form>
<strong>The Net Income of Balfour Betty plc is </strong><span
id="NetIncome">XXX</span>
<strong> million</strong><br />
<br />
<strong>The addition to retained earnings of Balfour Betty plc is
</strong><span id="AtRE">
XXX</span><strong> million</strong>
<br />
<br />
The first problem: rounding numbers. The following answer: <span
id="NetIncome"> needs to be rounded dynamically. I've tried to add a new
variable called RNetInc and add the following equation RNetInc =
NetInc.toFixed(4), but it only gave me two decimals, and after a refresh
it doesn't even work anymore. What is the best way to round the answer to
N decimals?
The second problem is one I don't know if it's possible. What I have in
mind is the following: A dropdown menu
<select>
<option value"1">1 decimal</option>
<option value"2">2 decimals</option>
<option value"3">3 decimals</option>
</select>
So, what I want is that when I click N decimal, the answer will change to
N decimal. This is a very complex situation, but one I often need.
Since I only know the (very) basics of Javascript, even using Google I
cannot find the answer. Can someone get me on the right track (if it's
even possible)? Thanks in advance.