I actually never really did the 10-15% rule, just kind of eyed the rats I bought. When I started to weigh the rats they always ended up around 10% though. I heard some people stop using the percentage rule after their snakes reach a certain weight?

You all make very good points. I think what I'll do is keep the rats around 70-80 grams, and if I end up with one thats a little larger, then at least I know she'll be able to handle it.
Thanks