How common is it to get pregnant from precum on finger if inserted?

Not common: Not impossible but certainly not common. A better birth control plan for you should be in your future if you are not ready to have children.

Don't know %: 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/