Magic Defense Bonus+ Magic Damage Taken -% = ?

Alright so I've recently rejoined FFXI and as such I've been mulling over gear for my paladin. But there's a question I can't quite figure out. I know magic defense -% caps at 50% with out aegis, 87.5% with aegis. Now where does Magic defense bonus come into play? I was thinking of trialing a water sword (+11 magic defense bonus, +11 mind) for a hold-me-over till aegis to macro in for nukes. Would this help me lower magic damage taken beyond the -50% flat magic damage taken cap?

If no how would I calculate magic defense bonus gear?

It will, but it's unnecessary.

100/(100+MDB) x (1-MDT)

I see, thank you austar. Just to double check my math, with the sword only +11, and assuming i have a mdt of 50, that would mean i would reduce all magic damage taken by 55.9. Which seems rather trivial... Hm. Thank you.

Not trying to be rude but you may want to give this a quick read. More than a few threads that explain mdt vs mdb.

A it would essentially be an additional 5% but if the mob has a abundance of MAB (through a buff say) it would be considerably less than that.

MDT is the most potent form of defense against magic damage. MDB is just icing on the cake. If you can find some MDB on other gear you'd utilize on another set its well worth combining the two, otherwise unless you have ample room MDB alone pieces are often the first to get left behind. Affronter Legs are a good piece because they also work well in an enmity set.