[lorinda]

The better you get at the game, the better your guesses will be.

For instance, there is a flush draw on the flop, you bet top pair, and get raised by the button.
The books cannot tell you more here without knowing the sort of player you are up against, some players are far more likely to raise this than others, so there is a probability that he has a flush draw.

You must weigh up the factors, if a guy ALWAYS raises in this position with a flush draw, then you need to assume that he either has a flush draw or a strong hand, from this you can make an estimate of what the % chance is that he has a flush draw.

A lot of the time you will not actually do the calculations, but if you read the books, they give you a feel for when a call may be right in a strange situation.

One of the classic ones is the semi bluff.
Often the small chance that a guy will fold by semi bluffing puts the value of the bet from negative (calling) to positive (betting) even if there is only, say, a 25% chance he will fold.

Use the numbers the books give as guidelines, there is not such a need to actually assign a value, just understand which sort of values affect these plays.