So here's the question:

Given a small collection, how do you determine how many breeder rats you need to feed 'em? Lets say for example, a weekly feed consists of a small number of a variety of sizes, something like:

2 weanlings
2 smalls
2 meds
1 large

How would you go about estimating the breeders needed to maintain that weekly feed, especially given the difference in time to raise for the different sizes?