if you don't want any headroom above, let's say, 105 dBSPL, you just have to calculate the power you need according to whatever SPL you want. but it's always a good thing to have some spare horsepower.
you can find a calculator here: http://www.crownaudio.com/apps_htm/designtools/elect-pwr-req.htm i think the calculator is calibrated for large halls and mainly outdoor events, so the results will not apply exactly to indoor needs. Less power is required indoors than outdoors.