Every 1 degree more of STA is worth about 1 cm less of saddle setback at average saddle heights. Even over a fairly broad range of saddle heights, the plus 1 degree to minus 1 cm relationship holds up pretty well. When there were no zero setback posts, you had to have a 75 degree seat tube angle to get forward enough for time trialing and triathlon type riding. Now you just ride a normal STA and use the zero setback post. I guess some folks would need both, though.