Thanks to you I'm not sure if shieldskill is even being useful against penetration
With 6 or 7 shield skill, I've had the following experiences against +3 arbalests with +3 steel bolts (100 pierce dmg):
19 shield armor (+3 Heavy Round Shield)-Frequently penetrated
21 shield armor (+3 Hand Pavise Shield)-Occasionally penetrated
25 shield armor (+3 Elite Cavalry Shield)-Rarely penetrated (once or twice with max speed bonus at pointe blank range)
35-37 shield armor (+3 Knightly kite/heater)-never penetrated (except by ballista)
So assume speed bonus could get the damage up to 125 pierce, that would make the minimum required shield armor to avoid penetration ~32, which jives with my observations, and the original equation in the game mechanic thread (which doesn't include the shield skill).
I wish shield skill affected penetration, because that would open up a bunch of shields for consideration. But right now if you want a shield larger than a buckler with 100+ speed that won't get penetrated by bolts, you have two choices. Pretty boring. It's a shame that some skills (shield, riding, weapon master) are such poor investments above the item requirements. I really wish they would buff these skills, because right now you have lvl 34 guys running around with specs like 30/18 and all their points in power strike/athletics (because other skills are so weak in comparison), and they are unstoppable juggernaughts.