Basically the "in a vacuum" Idea is where you should start when deciding what play to use in common situations. You are calculating Expected Value
. It is actually pretty simple. (do this away from the table, with real factual numbers whenever possible):
(outcome A * % it will happen) + (outcome B * % it will happen) = Expected Value.
A simple example is light 3-betting. In this case your opponent is opening from the hijack position for 3bb, and folding to 3-bets ~90% of the time. If he does not fold, he 4 bet shoves all in. You fold 100 % if he shoves because this is a bluff. In theory, if you raise him 3x, you win money every time you raise him EVEN WHEN YOU GET 4-BET and have to fold. Here is how it breaks down:
((3bb raise +1.5bb in the blinds)*.9) + (-9bb raise *.1)
= 4.05 + -.9
= +3.15 bb on average every time you make this play.
This is a starting point. Sometimes you have to make a different play.