My thoughts are that the minimum amount for a 6mth is 600ml which amounts to about 20oz, so I do 2x 8oz and 1 x4oz. As long as she takes that I'm happy. Sometimes she will take as much as 4 x 8oz bottles, but that's not the norm these days, though it was at first. Normally she takes about 3x8oz these through the course of the day now.