I guess I should have answered your question, which is that it's generally considered to be in the same plane as the top of the top longeron. That's been arrived at by the fact that if you buy an RR or Revmaster cowling, that's where the center of the prop hole ends up, blah, blah, blah...
Mark Langford, Huntsville, Alabama N56ML "at" hiwaay.net see KR2S project at http://home.hiwaay.net/~langford
