at what weight do you move up tub sizes?

I want to move to a rack system but don't know what sizes I need. The racks I'm looking at come in 28 and 41q tubs. What I want to know is at what weight do you move from 28 to the 41. Thanks

28/34 should be find for average sized male adults, and most female adults go into 41s. If you have a lot of female babies/subadults I'd probably just go for the 41qt. As for moving up I found that as long as the tub is cluttered babies do fine in larger tubs so you'd probably do fine sticking them in there right away.