DPO? Blue dye? Red dye?

I had a MC just over a year ago. My SO and I have agreed we will no longer “be cautious” in other words we are not intentionally trying but trying again. I was looking back at my old hpt pics and I just can’t remember how to count the DPO. Reason I ask is because right now I am 4 days late and I have either been early or right on time the past 6months when it came to AF. I have tested with the clear blue brand and both are reading negative. I do remember testing with red dye last year but didn’t get a good line until 15 days late. At this point I would like to know the DPO calculation... is it the day glow app predicts your ovulation or is it the first day ovulation begins or the last day of ovulation??? Help please...