my 2 previous cycles I have O'd on CD 13 & 15, I got a coverline and O day on day 14 this chart only for it to be taken away when I got another patch of EWCM, at which time I then raced out and bought an OPK which gave me a +ve for day 19 and then a -ve each time after then, hence new O day on day 19. I think ff has given you a coverline because of your ewcm and temps but if you keep using your OPK you may find that you get another patch of fertile cm and then if you also get a +ve OPK and a temp rise then ff will probably change your O day and coverline. (Hope I didn't ramble too much )