If my tail is too short, I start over. If my tail is waaaay too long, I still start over. But if it's only a little bit too long, then I just cut some off and then weave it in later.
You never, EVER want no tail at all because then the slip knot might unravel. I try to have at least 3 inches of a tail when I'm finished casting on.
I have yet to find the "perfect" method of estimating exactly where to place my slip knot. I just wing it and hope for the best.