When should I test? Unsure when AF is due.

Glow has predicted AF on 4th Oct (dont know why) but I haven't had a regular cycle since being off the pill. I used the Maybe Baby ovulation test and I had bits of ferning on 17-21 Sept, and what I believed was ovulation cramping on 18th. How many dpo should be a sure thing? I can't pin point the ovulation or confirm due AF so I'm in a pickle.