Cycle length unknown

Lindy • Happily married since Dec 2012. TTC #1 since Feb 2014. Found out on 6/13/14 that I am due 2/15/15. We are overjoyed!
I just realized tht I have no idea how long my cycles are. I went off bc in feb, got my first period (didn't have one while on bc) 3 weeks later, etc. For some reason I thought I had a 29 day cycle and AF is supposed to show today, according to that. Is there a way you can input an unknown cycle length? I have no idea when to test...tested this morning, bfn. No AF yet either.