Pre-ejaculate is usually just a drop or two, so it usually does go unnoticed.
Women can get pregnant from pre-ejaculate. However, if you had urinated since ejaculating last, that's good.
We can't give you an exact percentage or anything here because pregnancy depends on so many factors. However, I would not bother worrying too much if I were you. She can take a pregnancy test if her period is late, but it sounds like you two will probably be fine. Her chances of pregnancy would be pretty low from what you described. Just be very careful to not make this a habit in the future unless you two get some birth control.