Scatter, there are a couple of errors in your post: Defend definitely adds points to your shieldage. I don't know at which shield skill it starts, but it's a maximum of +2ac (added to the +2 from defending)
Light shield gets +1ac at each 25/50/75/100 so it maxes at +4ac, which gives +6ac when defending (that's the way the engine tells you: +10ac shield, +6ac from shield skill, just when defending +4ac from shield skill otherwise).
Iirc (I'll try to check) those skill values (25/50 etc) go for the other shield as well, capped at the ac for the shield in question (so you need 75 to get the +3 for the kite, etc.)
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