Hey, I'm no experrt first off, but I've done a few bigger beers and incremental feeding is a breeze. From what you've said, just wait til the yeast activity starts to slow a bit and add some sugars/fresh wort/etc. Don't worry about oxidiazation, at least I've never noticed any when doing this... just dump in some food for the yeast. I don't wait til the yeast are about pooped out either, feed them while theyr'e hungry and active.