All ballasts are not equal, but I use 36 watt bulbs in my AH Supply fixture which is made for 55 watt bulbs. It works fine. As far as the lighting goes, it isn't any where near an exact science. Whatever method you use will just give you a good starting point for what you need. You can also assume the light intensity falls off with the square of the distance, and adjust according to that. It will be fairly accurate if your reflectors aren't very good. If you had perfect reflectors you would get very little drop off in intensity with distance.