Across many OBATs: the Russian 57mm S-60 cannon is modeled with 17 penetration. This seems excessive for an AA gun. I can't find any source to validate that; nor did the Russians develop an APDS round for the 57mm antiaircraft gun. (Information on 57mm ammo is hard to find, but
the Russian Ammo Page is one source here.)
With the current values the ZSU-57-2 and the S-60 become amazingly deadly AT pieces in that regard, being able to tackle medium tanks. I suggest that somewhere between 7 to 9 seems like a better value.