I use needleless syringes all the time, I don't worry too much about the excess in tips. Pull some air in and it'll bring up whatever was in the tip also. The amount is very very minimal, just so you know, and generally won't change the end result TOO much. This depends on concentrates and flavours being used of course. The flavours where one or two drops will make or break a recipe generally aren't being measured in the ml/30 in my experience.
You could get a graduated cylinder to measure the difference, but if you have one you'd probably just be mixing in it rather than mucking about with syringes.