Please explain in more detail why you think it would be better done in JavaScript.

You have to process the form anyway, so you need a server-side script.

It's better, easier and more secure to add a line to your server-side script than it is to do it in JavaScript, plus there will be users for whom it won't work, as pointed out by other monks.

Please tell us what's given you the impression that JS is better for this task.

Apart from anything else, you should Be Afraid, Be Very Afraid of JS's ability to do floating point math. It has all the problems that Perl does in coming up with 99.9999999997 where you or I would come up with 100, but it doesn't have all the built-in functions that Perl does to take care of such problems.
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“Every bit of code is either naturally related to the problem at hand, or else it's an accidental side effect of the fact that you happened to solve the problem using a digital computer.”
M-J D