We went with lights about 50-60 feet up (in fact, the lights are about 5 feet from the top of the poles, because that's as far up as the cherry picker could go). We have four banks of five lights, but actually, we only use four of each bank because it gives enough light and doesn't overload the system. The key is spending a little time with the cherry picker and folks on the ground identifying that the entire field is covered, including the corners of the end zones.
And while you're running wire, go ahead and put in speakers and speaker wire (you can get away with one good speaker on each side of the field).