Going back to work... how much BM to feed?

I'm going back to work starting May 19 and have built up a bit of a stash for my MIL to use when she starts watching him, but I have no idea how much he should be getting once I go back. I'm planning on pumping at work so I guess on the first day, whatever I get out might be a small indication of how much he's getting per feed, but I know pumping isn't nearly as efficient at extracting milk as a baby is. I was thinking of starting with about 8 oz. for each feeding, is that too little?