I ran into the same issue with Tyson with the Lg adult mice before switching him to rats. I would shoot for the 10-15% rule, but if it's a little under 10% I just rolled with it and he was fine.
I would weigh and sort the mice when they came in. I would put 3 or 4 in a zip lock bag and label them. Then put all the ziplocks into a zip lock gallon size freezer bag and then into some grocery or Wal-Mart bags. This helps to keep them fresh and easy to find the size I needed.
When it came time to double up I would start using the 18-24gram mice and it worked out fine.
I found with perfect prey I got a LOT of mice in the 18-24 range and VERY few bigger than 27 grams. In two orders with them I only received one mouse bigger than 29 grams. I wasn't too happy with that, but they do deliver a great product, beautifully packaged for a great price.
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