Well, i don't think it is about doing it the hard way or making live easy. (In my book, by the way, there is no hard way. The exact way is easy enough.)
Originally Posted by Dan Fromm
The question was a simple one: whether the inverse square law applies.
It does, of course.
The excursion into the matter of focal length vs exit pupil position was a bit unfortunate, i think, because whatever you pick, the answer about the inverse square law remains the same.
Having said that...
Using a pocket calculator, the procedure to get to the absolute correct result requires no more than 13 key strokes, plus the input of two variables (where it says "extension" you need to enter the added extension, i.e. over the extension required for infinity focus. Where it says "focal length", the exit pupil position is required).
Figuring out the exit pupil position (one value) for each of your lenses is not much work. And with that, you can forgo any rough and ready guesstimates and always be spot on.
But i too am lazy, so my 'easy way' is to use tables. You only have to compile those once, and they'll be good forever.