I second Dennis's reply, in fact he said what I was thinking before I read his post! Both on the "hey kid, you just GOTTA use 15%", and the slightly leaner setting. Also you should run an engine on the same % of nitro for the beggining to the end of it's life cycle, except for a REALLLLLLY old one, sometimes it helps to go to a higher % at the end.

Jetts