The standard position is in the back corners blowing toward the middle center.
Although other variations are back corners blowing toward eachother, one back corner and 1 front side blowing toward eachother, two in the back corners blowing toward the middle and one blowing across the back, one blowing toward the center at the upper area of the water and one blowing toward the center mid way down the water level, etc.
It honestly depends on your setup, your live rock layout, your corals.
When I had a 20H going I had a little hang on back filter with out the media moving water plus two powerheads in the back corners blowing toward the front. The concept is to provide good water flow and random currents.
You should look up the manufactuers specs on the skimmer to size the pump.