First day of period? Do you count from spotting or first day of flow?

Guys I want to know what is considered the first day of period? I usually have less than 12 hours spotting and then full flow. This month I had 2 -3 days spotting then full flow. Should I could day one from spotting or from the day of full flow? It makes a huge difference on prediction of fertile week although I will still be doing opks. Thanks 😊

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