How accurate is this?

Destiny • 🤵🏻👰🏻👼🏻👧🏻
I've been logging my period on this app for 3 years now, and I almost always get my period when it says I will. How accurate is the 'fertile window' prediction? My husband and I haven't used birth control other than withdraw for a few years & he finished in me on the 30th when I had a 27% chance of becoming pregnant.