I'm am going through this exercise at the moment with my new baby pied female.

I weigh all her prey before feeding and record it on Degei. The plan is to monitor how much she eats over say the next year and once she hits about 1000g to weigh her after her poo. From there I can calculate what % of prey has been absorbed....

To be totally exact I would need to weigh the poo and the pee each time, but I don't particularly want to do that