e.g. based on how much you are risking VS the likelihood of winning the pot?
I know you don't know what cards the other's have, but you should be able to come up with some estimates based on your hand and the community cards.
Of course. Again, this is standard stuff. If you're not putting your opponents on hand ranges, considering pot odds, future pot odds, chances of improving hands etc, you're playing very poorly.
Obviously in No Limit games a big better would blow your formula up and you would fold.
Not at all.