a good question, there is probably a better way than this, but:
in bsp, in the front/side view, line your cursor up with the point where you want the platform to stop (at the top), then in the status bar at the bottom make a note of the second number after the '@' symbol. so it might say something like '@ 128, 64'. Then do the same with the point where you want the platform to stop at the bottom. Subtract the first from the second, then you have your number to put in the entity (dont put a minus though).
- Dag