You rented a specialized scale, right? A standard scale won't be sensitive enough. The most common one for pre and post-feed weights is the Medela BabyWeigh. The way I was told to do it was to put baby in a clean diaper then weigh them, zero the scale, feed and - without changing the diaper or clothing - weigh again. Average intake for a breastfed baby is 25 ounces (750 ml) a day with a typical range of 19-30 ounces (570-900 ml) per day.