I don't think it's weight based, more of an "average amount". Medical guidelines seem to say that babies need a minimum of 20oz a day from 6 months once on solids and that can include the dairy in their solids. That said milk needs to be the main source of nutrition until much closer to 1, so the more liquid milk the better really. At this age most babies will be taking 24-32oz ish and then are getting hungry enough to have some solids on top of their milk. You don't want them to start dropping milk in favour of solids until they get to around 10 months then most will drop to 3 bottles a day so thier intake may drop a bit.
HTH!
Laura