I had a scale that I used to weigh the baby before and after the feed to see how much she drank from me. It helped to give me peace of mind. I don't have much advice, other than to say it was a total leap of faith when I stopped supplementing. But I was only supplementing a few ounces a day at the time and baby was only drinking about 12-15 oz a day (she was only 2 weeks old and was small).
Good luck.
Jennifer