Same as Deborah. I go by size and age.
Most people go by 3 years, 1200 grams. I go by 3 years, 1500 grams.
In my experience as a breeder a 1200 gram female is going to waste your time. Most will lay 3-4 eggs and they are way more prone to becoming egg bound also. I'd prefer to breed a female that is at least 1500 grams because if you do only get 4 eggs, more than likely they will be 4 huge eggs.
I personally stopped cooling. The first year I ever bred I cooled and a couple of my animals got sick with RI. So after getting everyone back to peak condition I stopped cooling and just started pairing when my "season" started and my females still ovulated. Everything still works the same as before, I just never have to drop temps.