I'd use the value printed on the syringe, it should be there. If not, it should be on the box, or package insert, or the catalog you bought it from. Now, if this is a class, you may not have access to these other printed sources of error, like you would if you've ordered these supplies yourself, but you should ask to see them. After all, you were asked the question in lab write-up, so it should be answerable.
I can't say for sure, but I don't think your estimates are valid ways of determining error in the syringe.