Wrong cycle days

 I'm starting to think my total cycle days could be off and that is why I haven't had a BFP, yet. I originally stated 28 days ( assuming) so my expected AF is May 18th. This is my first cycle since getting off the IUD and getting my first period so I don't know exactly how many cycle days I have. Could I go by what it used to be before?? If I remember correctly my AF would come around the same date every month so I guess that would make it 30 day cycle, right? Anyway, I changed my cycle days to 30 so now my Expected AF is May 20th. I guess there's still hope?