I learned, as a rule of thumb, that you start at the foul line and take as many upright, normals steps you normally use in your approach, and then add a foot or half a step (depending on your size) for the slide - that's what is supposed to be a potential starting point.
Anyway, you might just use trial and error to find a good position. If you know that you have lots of space left to the foul line, then just move your starting point that much forward and keep up your normal delivery, just blending out the foul line. If you become too foul-line-conscious, chances are high that you choke up you slide and end up off-timing.
If you have a good routine, ending up very close to the foul line (and repeating it) is no problem. I wondered about this, too, when I started bowling, but after a while it became automatic, and in my whole bolwing career so far I only made 3 fouls. The less you worry about it, the better for the execution
--------------------
DizzyFugu - Reporting from Germany
Confused by bowling? Check out BR.com's vault of wisdom: the unofficial FAQ section