It's all in the cards!
Henzell says...
  1. Adjusted shield evasion penalty. Affects melee {accuracy} and {attack speed}. Its value is base_penalty - shield_skill/(5 + {size factor}).
  2. Base penalty is 1 for bucklers, 3 for shields, 5 for large shields. Size factor is -2 for Og, Tr, Na, Ce; +2 for Ha, Ko; +4 for Sp; 0 for others.

Information based on master branch and the great LearnDB.
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