Cycle length

This app says I should be ovalating around the 7th/8th September. As my cycle had been every where since coming off the pill my last cycle was 29 days according to this app. When I put on google ovalation calendar the ovalation time is different to the app. With my cycle getting back to normal could there be a chance of me ovalating 10-16 days after my period started like normal people do or could I still be later ovalating?