I was taught never to let the chick's crop completely empty. I like to think of the chicks as children, if you may. If my child were hungry, I would feed it. Same goes for the chick.

I'm going to assume you already know how to handfeed so I'll bypass all that. However, your breeder probably knows from experience how much your chick should be fed each mealtime so I probably would not go past 18 cc's if the crop appears to be full. IF the chick is still chirping and you can see that the crop is not yet full, add a little more. However, it's MUCH safer to slightly underfeed than overfeed. An overflowing crop can lead to aspiration/death. Just stick to frequent small meals. Good luck! I loved handfeeding my chicks. It was such a rewarding experience!