fertility window?! help!

Jessica
Hi ladies! 
This post isn't ovulation strip related but I'm hoping some of the ladies in this corner may be able to help me out. 
I'm not TTC but I recently decided to be more aware of my fertility window and learn more about my cycle. I'm still a little confused as to when my exact fertile dates are. I've heard some women say there's no way to know the exact dates and I've heard of other women having they're fertile days down to a science. 
I know Glow gives you a fertile window based on your cycle and it made me curious if those are really my exact dates. I thought it would help to download. Few additional apps just to see if the fertility windows linked up and now I'm even more confused than I was before. ALL apps have at least 6 months worth of cycle tracking on them and they all produce a similar period prediction but none have the same fertility window. 
Any ideas on how to get a more accurate understanding of my cycle? 
Not sure if it makes a difference but my cycle usually range between 25-28 days and the period prediction is almost always spot on. 
Sorry if this is in the wrong forum, thanks for any help!