I've never bothered with the formulas, and couldn't tell you the FOC of any set-up I have ever shot or currently shoot.
I just know what works for me, and what I see at tournaments--from the worst to the best. You'll see the ones that are really focused on score shooting lighter arrows. That was (is) one of the big selling points with metal risers and some ILF limbs--you can go as light as 5 gpp and not blow them up. Same goes with wheel bows--that's one reason there is an IBO speed and an AMO speed rating--one allows a lighter arrow, and that's one of the benchmarks the bows are judged by.
I'm really simple with it. I shoot what I like, and what I can shoot accurately. I listen to folks (one in particular) who has a ton more experience than I do, and who has proven himself in the field and in competition. And, of course, I use my own experience--works better than a calculator for me.