Starting Hand Theory

I'm a poker learner eager to understand the logic behind various poker concepts. Can someone help me with the theory behind starting hand requirements.
My guess is that at a 10 man table if you are first to act you need to open nothing but the top 10% of hands.
Likewise first to act with 4 players behind you you need top 20%. IOW, ratio of ranked starting hands over players still to act. Am I right?
Also, is there any mathematical concepts behind the decisions on re-raising, calling etc. after players have entered ahead of you?