I do division this way, in the form that i use I have 2 fields one called "ounces" and one called "pounds" and I put this in the custom calculation script box.
1// Obtain the value from the first field
2 var v1 = getField("Ounces").value;
3 // Obtain the value from the second field
4 var v2 = getField("Pounds").value;
5 // Set this field value equal to the difference
6 event.value = v1 / v2;
it pulls the value out of the ounces field "v1" and then the value from the pounds field "v2" and then divides them and returns the result.
Hope that helps,
Mark