I have just recently found a rat supplier that can supply me with all different sizes of rats, but before that I followed this, based on my own experience with ONE other ball that I got a few months before the other 14.

<200g = 1 mouse
>200g <400g = 2 mice
>400g <600g = 1 wean rat
>600g 1 small rat

Haven't gotten to the next stage yet... and probably wouldn't double anyone up until they got to around 2000g. I haven't had any refusals for those feeding on the rats, and have experienced awesome growth. I should be breeding a few 05's this year in November if things keep going this strong.

Of course, I have been warned about "teenage angst" that may put them off feed @ around 1000g... but I only have one ball that's over 1000g, and he's JUST over 1000g.

Keeping accurate feeding records of dates, weights, and what you're feeding, you'll be able to go back when you get new balls and figure out a "guideline", I've found.