If you hold him up after feeding him and look at his belly, you should see a white 'line' which is the formula in his belly. When you go to feed him next time, that line should be pretty much gone, if not completely. If it isn't, check again in 15 mins to half an hour. You should always wait until the milk line is gone before feeding again so you do not have issues with digestion
(I copied and pasted this from "Pappy" from another post)
I was looking for a whole seminar that the admin gave all about pinkies and furbies that I used for mine. I'll try to find it and send it to you.