I prefer to wait till 14 weeks for females. Males start whenever they feel like it haha. Like many others I do not weigh but I do make sure the female is dense feeling with a strong muscle mass.

Off topic: While fat does have a roll in pregnancy I feel that protein should not be ignored. A female with a lot of fat in her diet may be able to have one large litter well but her recoup time will be longer than a rat given the proper protein. Chicken (skinless white breast) is a small special treat for our girls that pop out 15+ back to back. We get chicken and beef bones from a butcher and the rats chainsaw right through them. Bones are not fed to rats that are not in breeding.