When on the hoods with my forearms horizontal, I like to have my elbows just barely in front of my knees. So that's the amount of stretch I like.

I've done many coasting studies on long steady grades. First of all, I emphasize safety when descending. We non-pros aren't doing it for money on closed courses. So I have my hands deep in the drops with 2 fingers on the brakes, forearms approximately horizontal, chin ~2" off the stem, back flat, knees gripping the top tube, and elbows pulled all the way under my chest, as far as they'll go. On bumpy or technical descents, I lift my butt slightly off the saddle. Testing against other similar riders, this position is as fast or faster than the usual hands on tops that one sees, plus it's safer. On the same grades, I'm slightly faster on aerobars, but not much, maybe 1 mph at 40.