When should I start OPK when using clomid?

In all of the excitement of getting the hope back that I'm able to conceive, I didn't think to ask my Dr when I should start my opk while using clomid. Should I start the day of my last dose or should I go by what the opk directions say and go by my cycle length as to when to start testing. AF is due on the 21st and I am suppose to start my treatment on CD 3-7. If I go by my cycle length I should start my opk on CD12. So many questions in so little time. Any advice?