Well I dont know the % but there are thousands of sperm you're dealing with so there could be a pretty good chance, tons of girls get pregnant while using the pull out "method"
its not a form of birth control, it doesn't work lol if I were you i'd just get a test thats your only way of finding out.
And precum is.. Cum that comes out of his penis when he gets excited, sooo when you're going at it its on his penis and it'll do the deed. Be more careeeful next time. How old are you?