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As far as I know, you've got the math right.
Manashield ignores all def/prot so armour isn't useful to wear with it. I think the mdef only affects magic attacks, so if the 100 damage is from thunder you'll take low damage. If you are shot by an arrow only the manashield efficiency will affect it.
Manashield is very useful for Tech Duinn's Geata boss. Cuts the damage from 1k+ down to ~300.
Reforge does not stack as far as the wiki says. Lv20 reforge will give 4.0 efficiency. With some math, that makes manashield reforge useless if you have more than 550 magic attack.