Originally Posted by baldilocks
After a couple of batches you will get an idea of how much wort you end up with after a certain length boil.
I think it can really only be gauged corretly through experience. Take VERY good notes on the next few batches. Be precise with your water levels and exactly what temp you are boiling at. I built up to 90 minute boils by messing with my temp and lengths of boil (started with a high setting and 60 minutes on a stove top...now I can get a good boil at 8 through 90 with predictable results.)
Always aim low...you can add a cup of water at the end, but I really hate throwing out any hoppy-wort goodness.