Equity question

[SeanC]

Hi,

What are the general rules of how to choose the right action based on your equity in the pot at that moment? It makes sense to normally raise when your equity is over 50%, as you would technically be happy to get it all-in under those circumstances. But what if your equity is less than 50%? When Pstoving hands, the equity is based on the turn and river being dealt, right? That being the case, isn't a simple EV calc based on your equity wrong because you could be facing another bet on the turn (if you're calculating flop equity). Or is the following simply correct:

OPP bets $100 into a $100 pot. You have 40% equity against his range. EV=(300 x .4)-100=$20, make the call.