How are you going to convert speed to height if you're already at Best Glide Speed? Surely by slowing down your descent angle will increase (apart from the small yug effect when you pull back; "momentum" in a bug smasher??)? Now, if you were deliberately descending at faster than Best Glide, I could understand adjusting the aim point by slowing down. But if you don't have the capacity to slow down because you're already at Best Glide...