On first day off period, and only pre cum, what is the chance of pregnancy?

It depends: The only right answer would be a guess. It all depends upon when you ovulate in your cycle. Pre-cum has the same fresh sperm as ejaculation does. Sperm cause pregnancy. If you are ovulating then, then you are at risk of pregnancy. Know when you ovulate!

Very low unless: period is long and menstrual cycle is short. Most studies have shown low levels of sperm or no sperm in pre-ejaculate with sperm not being motile (however, don't know how promptly samples were examined). One small study published in 2011 showed that there are some individuals with a high motile sperm count in pre-ejaculate. See: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3564677/