OK, so the light beam goes thru about one inch or so of graduated overlapping subtractive filters with a
range of maybe 175 cc's. No gearing or dial on conventional equip is going to give incremental control
on such a small surface as you imagine, but you should be able to dial it in with less than a 5cc error. With a very sophiticated feedback colorhead like I use, one can sustain control within about 1cc. Gels (at least the ones I've measured on a densitometer) are often plus/minus around 4 cc's or worse. The pale ones tend to be far worse. But I have no trouble switching filtration even on my old Chromega additive colorhead in the dark, like for split printing. Keypads are nicer, but still no big deal. I've done a few experimental color prints this way, but if I really had to correct a print I'd resort to selective pan film masking of the neg, but that's another story. And the Fauxtoshop crowd have their own way of doing it, but most of them probably don't even notice green faces.