Yup, always from the first day of your period hun. So if you have 33 day cycle then you would be ov around CD19

It doesnt matter when you should have gotten your period only when you do get it. So if your period started on Friday then thats day 1 no matter whether it was due the friday before of the friday after.HTH
My cycle has never been regular, varies for 31 to 40 days

In the end I had to start charting cos I never knew when AF was due