Both of my docs, the RE and the regular one, had me start when I got my BFP. BUT, I was not comfortable with that, so I had some pills laying around (the first doc prescribed two months worth, since I was pregnant and I managed to drag them out for a couple months of the TTW) that I took from day 17 (two days after ovulation) until I got my BFP and the RE prescribed suppositories.
And I also do not have a noted or diagnosed low progesterone problem. My levels were low in previous draws, but I was also losing the baby at those times, so it is hard to tell which was first, low progesterone or losing the baby. KWIM?
If you are uncomfortable with taking them later, ask if you can start them earlier. I mean, it is a difference of a week and the pills DO NOT have negative effects...so if it makes you feel better, I would think the doc would not care. Ya know? I hope you get it figured out