Hmmmm, I couldn't find a specific explanation of what the penetration total is for a spell cast using Wizard's Communion... Is it still just the lead character's spell roll, substracting the divided spell level? (if so, then I'm stuck with my group still using 4th edition rules where the level has no effect on penetration... Any idea on how to make Communion matters on penetration in 4th edition?)
Serf's Parma, but I think you're calculating the penetratioon correctly (that's how I've always done it). On the other hand, however, IIRC all the participants are considered to be the caster so perhaps it would be a better idea to simply use the participant with the highest casting total to determine penetration?