The "parrying a missile attack" rules are vague and somewhat contradictory. You have to reduce your OB, shifting part or all of it to DB, but it has its own activity requirement, which means you generally can't be making an attack which would give you an OB. Also, it's different from normal parrying in that you don't have to be making an attack, and if you are, it doesn't have to be against the same opponent.
It's possible the intent is that you spend 50% activity on parrying a missile attack only if you are not already taking an attack action (in which case you're already spending 60-100% on that attack). I'm not sure.
You'd get the regular shield DB bonus regardless.