I run 30% heli fuel and my main needle is never more than 3/4 turn open. With 15%, I'd expect it to be closed even more than with 30%. As was mentioned before, set your throttle curve to control the headspeed. I think I only need 65% throttle to maintain decent headspeed in hover on my OS 50 powered Evo and max in normal mode is only 80% throttle.
Matt