I know... math is evil!
I will be getting my rats from the reptile store where I got all my mice. Since they go by size there, I plan on asking for weaned rats and weighing them to get a rough idea of age by weight. 250 is the goal before putting the males in.
And I suppose you're right with the time span. I'll see if I can find some plump, healthy looking rats when I go in tomorrow to pick up the PK smalls for my big girls (they kill them for me since I don't have my CO2 chamber sealed up yet). I have two open mouse tubs since I fed off my other two trios that I can house the pairs in.
Thanks!