I STF'd again today and tested a new character. I took screenshots each time I raised HA, PD, or WPF (in 10 point increments) on my way to a 21/18 horse archer build designed to see if the yumi could be worth using. After hours of tedious testing and screenshots, what did I discover?
Not a lot.
It seems like I was unable to get an accuracy increase on any bows as I went over the PD requirement this time. Mild differences in screenshots could be errors on my part in spite of me taking a half dozen screenshots or more attempting to capture an accurate reticule. In fact, I pretty much always lost a little bit of accuracy as I increased PD above the requirement. It's as if on my last character something magical happened when I finally reached PD8 on a 24/18 build.
So my nonsensical conclusion is that PD decreases accuracy a very small amount unless you have PD8, at which point you get a nice bonus over someone who has PD5 and similar WPF.
And for horse archers, HA4 is definitely a must.
For now it really does seem like the standard archer builds are the best.
1. Skirmisher: 15 Str, 24 Agi. 5 PS, 5 Ath, 5 PD, 8 WM. Melee WPF.
2. Horse Archer: 15 Str, 24 Agi. 4 HA, 6 Riding, 5 PD, 8 WM.
3. Dedicated Archer: 24 Str, 18 Agi. 3 Ath, 8 PD, 6 WM.
After hours of work for nothing informative, I give up on further testing; it'd be nice if some dev would give us the formula.