I start by figuring out how noisy my fans are at certain speeds. For me, 50% is basically silent. So that's my baseline. Then I figure out how loud I find my fans to be tolerable. About 75% for me before they start to get "loud" by my standards. This is my "max speed" at comfortable temps. Then I scale up to 100% at a temp I consider too high.
So for me, fans are all bound to CPU temps. My GPU doesn't see more than 3 degrees of Delta from 50% case fan speed up to 100% anyway. So I run 50% up to 50C, then they increase up to 75% at 60C, then by 80C they are at 100%. But while gaming my CPU never goes above 60C, usually mid 50's. I prefer linear curves, and not stepping for fan speed. Major steps up/down in speed are annoying.
To be honest, unless you want to try to get as quiet as possible, just find the speed you find your fans tolerable and set it there all the time. Only going higher if temps get sketchy.