Originally Posted by
Carbonfiberboy
CADding it out, on a 53cm frame, changes in seat tube angle will change saddle position about 10mm for every degree of seat tube angle change, at least in the 71°-75° range.
Depending on what one needs in saddle position to achieve good balance, this may or may not be an issue. I'm a long torso guy who needs the saddle all the way back on a setback post. Some bike and saddle combos don't have quite enough adjustment to suit me, but I get by. YMMV.
So in your case, I guess you need a slacker seat tube angle than most. Do you know around what seat tube angle becomes too steep for you?
Relating this back to the OP’s point, you don’t really need the measurement he is describing, you just need to know the STA is not to steep. Do I have that correct?