Finding the caps is quite hard because each bow is different, higher PD reduces effective wpf but on the other hand, in some cases it seems to contribute to higher accuracy of bows with low requirements, then there is the armor penalty for 10 effective weight and more, an accuracy penalty for very fast turns, and so on...
Horse archery has its own penalty effect on effective wpf. If the HA penalty on wpf is 100% by default, it's reduced by 10% for each point in HA you skill. When the horse is slow or stands still, it is lowered, too.
Probably best to get a Masterwork set first, then transfer everything to a STF, and test the accuracy with different builds and wpf splits between melee and archery. Solving it mathematically is almost impossible, since there are many variables and parameters, most of them with a known qualitative, but unknown quantitative effect. So do it the trial and error way. If your buddy does not want to wait with the build design until he has a +3 set, ask a friend who plays longer to borrow the stuff for a while, or get it from your armory.
