Q2: How do I determine if a value bet (or raise) is +EV?

Answer: Compare the payoff to the chance of winning.

Note: Here the payoff is a function of how much you're betting. What is already in the pot is irrelevant.

* Example: No cards left to come

Heads-up on the river in a game with no rake, Villain checks to us in a pot of $250, and we have $200 left behind. We think that if we make a value bet of $200 that we'll have 51% equity against his calling range. Then our value bet is +EV because 51% of the time we win $200 from our opponent's call, and 49% of the time we lose the $200 we're betting.