Figuring out what to do in common situations

Yesterday I was thinking about my game, and I realized that I never really learned any numbers or percentages or anything, and that I just click away without thinking too much.
Because of this, I decided to figure out how to play many common situations, so at least I have some idea what I'm doing when I'm multitabling.

Take the small blind for example, when there are x limpers, in the early levels. I usually think 'oh well, i'm getting x:1 and that's probably good. Let's call', but I never calculated anything. With PokerStove you can calculate your equity with a certain hand vs a random hand (BB) and a certain range (the limper). You can also easily calculate your pot odds. Is this enough to determine when it's +EV to call? Of course there are implied odds too, and because I'm multitabling I want to avoid marginally +EV hands. What I want to know is how to calculate what a good 'default approach' is? I would really like to do this for many situations, but I don't know how. [img]/images/graemlins/blush.gif[/img]

I'm sure that many of the better players have spent some time calculating stuff and thinking things through. How and what have you calculated? How can I fix my leaks and make more +EV plays?!