When I was a FWB to a married woman many years ago the only rule was that there were no rules! That's why I was her FWB, because her hubby simply couldn't be bothered satisfying her sexual needs, and she had a lot of sexual needs!!! Suffice to say that I wasn't her only FWB! I liked that rule VERY much!!!
No random drop byes, scheduled meetings only. No personal feeling, just sex. Your significant other can be there if they want.