If you have three balls, 1.2 or 1.3 would be fine. If you think about it, that's about 20-30 babies every 2~ months. If you have small bps, and you have too many rat babies, you can freeze the rat pups before their eyes/ears open. Thus your overpopulation is solved.
On average I have 25 rats at any given time and only 6 of them are breeders/future breeders.