Help with theory

[MJL]

I've come to understand that if I have a pocket pair and my opponent has overcards I will win about 55% of the time. I have learned to figure out that if my opponent has overcards 60% of the time I multiply 60x55 to find that I will win 33% of these times.
My question is what is used to fiqure the chances of an opponent having certain hands or folding. Is their a formula or is it random?

Example in a tournament Im at 3800, less than my opponent and the blinds are 300/600.
I have a pair of tens in LP. My opponent open raises to 1800 in MP and I must figure out if I should go all in, call or fold.
I must figure out:
He has a higher pair how often?
He has over cards how often?
He has one higher and one lower how often?
He will fold how often?

The first question is: Is how often a good estimate or is it more logical than that?

Second:
After I come up with the possiblily of winning pecentage what do I do now. If I win 70% of the time what now? How does this play into EV.