There must be an easier way?

Hi,
I am trying to make a form that will calculate the total from a number of fields. I am using the simple javascript function below but the problem is I want to deal with up 200 fields..having an entry for eachone of them is going to be a pain...Is there an easier way of doing this?

function doMath() {
var one = eval(document.myform.field1.value)
var two = eval(document.myform.field2.value)
var GrandTotal = one + two
document.myform.amount.value=custRound(GrandTotal,2);
}

Thanks in advance
Mizzy