How to figure out ovulation when your period is irregular?

I used to have extremely irregular periods. I was only regular when I had my iud so it was easy for me to calculate fertile week and ovulation window. I had a miscarriage during my first pregnancy in February and just recently got my period again. I was supposed to be ovulating on Monday.... although I randomly

started spotting on tuesday. Part of me tells me to let nature do its thing and let time tell. But at the end of the day, I still wonder how do people with irregular periods calculate their ovulation days?