Cari,

Here's an explanation of seat tube angle.

http://www.billbostoncycles.com/seat_tube_angle.htm

Because I have a short femur I need to have a steeper st angle to be positioned over the bike properly and a zero degree seat post. My road bike angle is 75 degrees, the RacerX is 74.5 degrees. My new custom frame is also 74.5 degrees. The person who fitted my road bike said I could go to a 76 degree st angle.

And my saddle is also forward instead of centered.

Mountain bikes typically have a seat tube angle of 72-73 degrees.