I agree with this. I fit from BB->saddle->bars so seat tube angle is really the first factor in frame fitting for geometry (the seat tube length is a close second but that part is easy).

When I spec'ed out my frame I used my current saddle-BB position/relationship to figure out a good seat tube angle. My "methodology" was pretty simple - I figured out the angle of a straight line between my BB and the center of my saddle rails. This was 76 degrees, based on having a zero set back post. The builder recommended going slightly less aggressive at 75.5 degrees. This has worked out fine.

In the past my saddles were always slammed all the way forward. Now they're slightly forward of center, with plenty of room to go either way. I thought I'd end up experimenting with more forward positions, since those positions weren't available to me with slacker seat tube angles, but after trying a slightly more forward position I returned to what appears to be a pretty optimal position for me, that 76 degree position (give or take a few mm).

(In terms of seat tube length I knew I wanted a compact type geometry based on how a lower top tube bike felt when out of the saddle, and I just asked for the same size seat tube length as I had on that bike - 40 cm c-c, about 44 cm c-t. I knew I'd have plenty of seat post left so that wasn't a concern. With a level top tube I'd been riding a 52 cm.)