Periods after bc

Annie • Married 5 years & counting. Mama to a sweet boy and many 🌈's, baby #2 was a rough miscarriage as it was 7w6ds in and we had just days before seen our babes heartbeat
So I recently came off bc and am trying to figure out what days I need to be careful to avoid getting pregnant for now.   Before bc my cycles would range 28-46 days (pcos) so I'm not sure what they will be right now but if my cycle is on the longer side or is never the same how do I figure out the ovulation time?