I'm really not sure you can hard dictate twist rate for pellets. It seems as if the 1 in 16 or 17 is most common and there are plenty of examples of good and bad shooting barrels with a variety of pellet weights and calibers. I have Career 707 barrels in 22 and 177 that are 1in 22 and shoot extremely well with a variety of pellets and LW recently started offering a 1 in 30 that has had mixed levels of success. The diabolo form is self stabilizing and will even shoot fairly well with no rifling. In testing a pile of barrels in the last year in 25 cal, we ended up selecting some 1 in 30s as the best overall for 25 and 33 gn pellets. These theoretically will not shoot slugs and some of the 16s and 17s were very close and might shoot slugs. Anyway, the barrel internal form seems to have more to do with the accuracy than the twist from our testing and from my experience over many years of shooting ag's, so I'm not so sure you can formula-ize the twist rate for pellets.
I guess what I'm suggesting is that you look to the barrel finishing and not worry much about twist.
FWIW, I had a good time on Wednesday shooting some long range with my friends near Flag in fairly constant wind. I think I hit 10 of 20 on a 4 inch triangle at 200 yards and very consistent on a 3 inch spinner at 160 with my RW in 22 w factory 1 in 16 barrel. For some reason I had a tougher time on a 5/8 spinner at 60 but fun day for sure.
Hth
Bob