I am surely no expert and I only know what I have read about gliders, but I would think if you had a kitchen scale that you could weigh the joey on and can monitor his weight to make sure he is gaining from mama's milk he should be ok on what she can give him. If he is not gaining or even starts losing you should sumplement with the formula. Like I said I am no expert and I don't even have experience with joeys, but I do know the dr.s do this with human babies to monitor how much they are recieving.