How do you fine-tune sensitivity and weight?

So, I problem I have a lot - especially if I take longer breaks in games - is getting my sensitivity right (in both PC and console games). Obviously I can get it down to a certain range, but I always seem to pick around in there without settling on something. Can't be good. How do you precisely decided where it should be for you?

(Would apply to weight too, if my gear was nice enough, but I imagine others might have the same question.)