Calculate first day of period

Maggie

I'm 9 or 10 dpo, on Metformin and been spotting for 4 days now. It's been light and not enough to fill a pad or tampon. Tonight it was a little heavier and some on a pad but didn't fill it. Do I count that as the first day or when the pad gets full? I do temp and it hasn't dipped yet. I am going to try tomorrow, but my thermometer is difficult is getting difficult to turn on. Just want to get the days right so I know when cycle starts.