Not that effective. Btw you can get pregnant off of pre cum. So it's better to just wrap it before you tap it. 

There's a chance but we did this for a year and never got pregnant. Then we got pregnant the first month of actually trying. Yes, you can get pregnant from precum but if you really look into it and do research it is more likely to happen if he hasn't peed since the last time he ejaculated. Once he has gone pee everything is cleared out. (And be research I mean actual medical research, not just what you read online 😒)

Not very high, as long as all of his cum is in the condom and there is none in you then you will be fine