Actually, it isn't related to weight at all. Surprisingly the amount of bm drunk at different ages doesn't vary a lot, but the content does. 750ml is an average intake over 24 hours, so the best way to get an estimate is to divide 750mls by the number of feeds in 24 hours. It is not exact, as of course they don't drink the same amount at each feed. But they do also tend to drink less from a bottle, so it will leave you with enough EBM.

I always stored EBM in 40 - 60 mls lots to avoid wastage. Often a baby won't drink all (or any!!) of the bottle and it's heartbreaking to throw the EBM out. And of course, once it's warmed, you need to discard at the end of the feed.