That is the problem with bottle feeding I'm afraid, you do waste a bit! When he was breastfeeding he could well have been doing the same thing, some smaller feeds, some larger but of course you couldn't really tell. It may be that you find one particular feed is always smaller than the others so you can adjust but then one day when he wants more you will be making up extra formula
.
I would (an did) make the same size feeds - so if 8oz is the most he will take then make 8oz bottles. I know you will be wasting some from some feeds but at least it will be there for him if he wants it. With formula feeds there should be some left in the bottle at the end of a feed so they can stop when they are full and not just because the bottle is empty.
If you are still on a slow flow teat going up to the next size can help them take bigger feeds too. Are you doing a dreamfeed? How is his day eating routine?
Laura