@casportpony is the expert on feeding by tube or syringe, I found where she talks about how much to give on this thread. You can pm her if you are having difficulty with the formula, I'm sure she will be able to help you figure the amount you need.
https://www.backyardchickens.com/t/805728/go-team-tube-feeding/60
POST#69
"""Very smart question as that is not talked about in this thread. Also not mentioned is the importance of giving fluids first. That said, I can tell you that I start by giving just fluids until I'm pretty sure they are properly hydrated.
Total crop volume is said to be 5% of their body weight, but experts suggest giving 1/3 or 1/2 of that when starting out. When tubing one of mine I will start by giving a 1000 gram bird 15-25ml of warmed Gatorade or Pedialyte. Repeat in 60-90 minutes if crop ha mostly cleared, then at least one or two more times at 2-4 hour intervals. Once they are pooping clear liquid I will tube Kaytee baby bird food mixed at 1 part food, 2.5-3 parts water, and this I do 2-4 times a day. Tubing is part science, part art, so start with the minimum amount, and each time you tube, try adding a little more, but don't exceed the 5% by too much.
Does that help? If you need help figuring out how to give, let me know and I'll help you come up with a plan."""