chances of getting pregnant

i know plan b shouldn’t be used as a birth control on a regular basis but i want my bf to cum in me just once. what are the chances of getting pregnant if i take the plan b immediately after he cums in me? and/or what is the best time slot to take the pill to make sure i don’t get pregnant.

ALSO: I AM ON BIRTH CONTROL PILLS i take them almost the same time every day. we wanna use plan b as well as the bc pill to be extra safe. any info would be great