Ovulation Calendar confusion!!

Patient: Hi ..i usually get my period exactly every 21 days (3 weeks) and this time my period was 1 week late, i got it on the 28thday (4 weeks) i’m trying to conceive and i mostly use the ovulation calendar to calculate the days i’m supposed to try to conceive on but am confused on whether to put 28 days cycle (every 3 weeks period) or 33 days cycle (every 4 weeks period) to get the correct calculation. i really want to put the right date so please HELP! ;)thank you very much will greatly appreciate it.