ZDoom

Does anyone have any specific formula to convert my mouse sensitivity from, for example Source Engine games, to GZDoom/Zandronum?

There is none outside of you adjusting the game according to feel.

I use a mousepad, so I try to use that as a gauge as to how far my player should be turning when I move from one end to the other, and try to match it across all the games I play. Other than that, I believe it just is a matter of feel.