I personally think the whole weight thing is, to be nice, stupid

My reasons for this are because there are too many other factors that come in to play. Let's say you have a 9" female and an 7" female. Both are 55 grams. This doesn't mean that they are ready to breed, since the longer one would be much skinnier than the shorter one. Hope that makes sense
Anyways, go by appearance not by a scale. If you have females old enough (10 months+) and they have a good body size, thick tails and are regular eaters and shedders, then I'd say they are ready to breed. The most important thing is that they are healthy and active. This way you produce healthy active babies
Good question, btw!
Julie