It mainly has to do with your speed and the concept of driving up to build up speed and then playing with turning down to try to optimize your speed and VMG to the C mark. The ability to fly a hull downwind- just barely out of the water) has a lot to do with the weight balance on the boat, etc. In the end you have to find the sweet spot which maximizes speed/VMG. I think it is hard to define it as a 90 deg thing although I know where you are coming from with that as that is what a lot of books say. Play the feel of the course, not the defined 90 or 100 or 80 or whatever angle