Yep, this sounds much more like an electrical / electronic engineering question than something to do with lisp. You need to get the formula first, only then can you use lisp to calculate that formula.

I'm just guessing here (as I'm no engineer), but as this is a cross section - would a simple area be sufficient (i.e. get area of shape then substitute into the formula as S). Perhaps there's a lot more to it than that - and that's why you'd need someone better versed in such calculations.