archery wpf is specific case.
at first - every point in PD kill 14 points of your effective wpf.
So, if you have 100 wpf in archery and 5 PD - you'll receive 30 efective wpf in archery. Not much eh?
Second - you start to lose wpf, if you have more than 7 overall armor weight.
But - you must multiply weight of hand armor by 2, and head armor by 3.
Good luck.