I think you need to do e4d method - unprotected sex every 96 (better chance of pink) or 72 (better chance of conception) hours. This is still one attempt (BD every 8 days will be WAYYYYY too long and give you next to no chance of conception)
OPK can be false negatives so with those very long cycles I think you're better off having regular sex so you won't miss a month.