I've sailed with mine..
with the topping lift slack. If I crank down on the main real hard it just brushes, but normal sailing its about an inch off the top. Howeve in order to accomplish that, I have to keep the top straps firm and the center bow of the bimini almost flat. My middle bimini bow is adjustable, so I keep it flat so the center height is the same as the front and rear bow.