1. If the baby poops during the feeding and you decide to change the diaper, you weigh baby right before changing him, and subtract the beginning weight from the post-poop weight. Then you weigh again once baby is in a clean diaper, and proceed with the feeding, weighing again at the conclusion. At the very end, you add the pre-poop intake to the post-poop intake to get the total for the feeding.
2. Do at least a day's worth of before and after weights before making any judgments about intake.
3. Let's see what baby is taking in over the course of an average feeding before we get into that? I mean, if intake is good you can start weaning off of supplements. If it's not, then you have a lot of different options, but what they are depends on how good/bad the average nursing session ism in terms of intake.