Unprotected sex/coming inside

Greetings, everyone. Both me and my girlfriend would like to implement some sessions of unprotected sex (including finishing inside) in the course of our intimacy (I was thinking like twice per month). We are both clinincally tested and clean from any sort of STD, and she regularly takes the multiphasic pill (which, according to data, has a protection rate of 99%). I have been meaning to ask: does anyone else have experience with this kind of intimacy? How safe is it, actually? And what would be the best times in a month to have this kind of intercourse while avoiding pregnancy as much as possible? Thanks in advance.