period question

Brittany • Mommy to a wonderful 12yr old boy, 9yr old boy 7yr old girl 6yr old girl and 3yr old girl. Due March 11th 2023 with a boy.
I'm trying to prevent pregnancy as I have a 19mth old an 6mth old. Anyways I want my period to be right on here. Do I start from the day I had break through bleeding/spotting or the day I actually start bleeding bleeding?