I hear 8” from the top of the water. I too have a center brace. 4?I think four will be just fine For a 72”. I only wanted to get three, but with my center glass brace, I didn’t think mounting one right over in the middle would be good. I only have the inner two at 50% intensity, the outer two at 20%.
Another thing I liked was the low mounting height. Works great for my low profile hood.