My questions are :
1.) If i enchant some arrows with a level 25 PeCo effect (Base 20 +1 touch) which inflict a Incapacitating wound, presumably :
a.) if it hit's a magus, or a creature with magical might, then their resistance from form & parma would apply e.g must overcome their resistance;
if so, how do you determine the penetration total for said arrow? would it be 25 + stress die? Also which form would be used? He because it's wood, Te if the arrow has a metal tip or Vi because it's a magical effect? You could elect to increase penetration as per page 99 i realise
Thanks in confusion!
You have to add penetration to the effect as you would with enchanted items. The form used for MR is the form of the effect, in this case, Corpus.
Base penetration is 0. As Jonathan.Link said, anything beyond that needs to be added. Charged items can have quite high penetration though, since you only need to hit the level of the effect rather than accumulate points beyond it as with invested or lesser enchanted devices. I had a giant blooded archer verditius character in the Brunnaburgh saga with a bunch of charged item arrows with different effects. He was great fun. Also, turns out welsh longbows sized for an 8 foot tall man are really, really dangerous.
It is quite common for people running games to alter the rules for penetration when it comes to charged items. The rules as printed are, compared to other ways to get penetration, quite powerful. I change them for my games.