Are those 10 ml syringes? I go by the crop, feeding and checking the crop as I go (you have to wait a few seconds after feeding to check it). As SW says, it should feel like a 3/4 filled bag Kaytee...squidgy, not tight.
I have a page on hand feeding baby pigeon on my website, it contains what I have learned from my own experience and what I have learned from this forum. (If anyone wants to add anything to the information on that page please let me know. The more information we can offer new rescuers the more likely the pigeon is to survive).
This is the page on how much to feed.
I have found that the syringe and balloon method, or using a baby bottle works well, the pigeons that I have fed usually show me when they are satisfied.
This link describes it step by step and also has before and after feeding photographs..
This link is good because it has photos of the day to day development of baby pigeons that are being fed by their parents (you have to scroll) down. So you can assess whether your pigeon is developing at the right rate. It also has photos of well filled crops.