You can get ovulation test kits which measure hormones to see when you are ovulating. Amazon has some affordable ones.
Yah, I noticed the app suggests an earlier fertile window too. Generally speaking, if you know you're ovulating on day X, you should have sex as much as you can/want to for the 3-4 days leading up to day X as well as a day or two after, in case you predicted the wrong day.
Good luck~