letrazole fertility meds

For those that have taken letrazole or femera, what cycle day did you notice a positive opk? I've been taking it for a couple months now and I start opk on day 12 usually always get a positive on day 14. I take letrazole 5mg twice a day from day 5-9. I have been reading that it may cause false positives if testing so close to the day 9 and many ovulate later around day 16-18.