PP is always a matter of dminishing returns. Essentially, you get 99% on your 2nd play, all the way down to 1% at your 100th.
I believe the math essentially works out like thus, assuming your top play is the pp for all plays: Top play x 20 + Bonus pp for score submissions. If your top play is 200pp, then your max is 4416, thereabouts.
SR has almost no bearing on it, insofar as it's just a metric for how difficult the game judges the map to be. If you want better gains, you need to set plays that are at least within or over 10-50pp of your top play, depending on how large your spread is.
(also, wrong forum, technically. You'd want the mania specific one.)