That makes sense. So if the angles are the same then TT dimension should rule the day because the other points are fixed.
Exactly!
But that would then make a case for more manufacturers printing that measurement, because seat angles vary between manufacturers. The only way to know the relationship to the BB is to provide a number. Or for someone to break out with their sine/cosine calculations.
The best case for using setback is that it indicates a change in seat tube angle. If the MFGR is not going to provide STA, then yes, they ought to publish setback. One or the other is necessary to make a good decision about fit. You can back calculate STA if you have the setback and the STL. But if you get STA, it's really the easiest way to approach the differences between different design styles.
The rule of thumb is really pretty simple, for each degree of angle, you need to move the saddle about 1CM forward or backward. An easy rule of thumb to remember. But I also keep an excel spreadsheet with a table for calculating it, just for grins.