Seat tube angle is more about feel than anything, unless we are talking TT Bikes. Seat tube angle can change a whole host of things, handling is mostly from the handlebars to the front axle. TT length has nothing to do with bike fit. The reason being, BB to handlebar distance on a horizontal plane is the dimension that matters. If you change the angle of the seat tube, the TT length changes. If the BB is still in the same position & you are measuring the TT, you will be in for a surprise. Case in point,my MX leader has a 58.2tt & my Pinarello Montello a 56.5 TT. The Montello fits a half CM bigger. Put the frames side by side & the MX is a much bigger frame, real estate wise. However, bb to front end is that half CM shorter. The have nearly identical front geometry. The MX has a 72.2 deg ST, Pinarello 73.5