How far can precum travel down skin onto the outside of the vagina to impregnate someone?

See below: Sperm needs to actually enter the vagina or be deposited at the introitus for pregnancy to occur - whether from ejaculate or pre-ejaculate. If you use a condom every time and correctly - you won't have to worry so much about the distance that pre-ejaculate can travel.

Agree with Dr. F: Semen not entering the vagina will not result in pregnancy, but contact with skin around the vulva can still transmit infections.