Accurate Ovulation

Hello ladies. I should probably know this but I don't. How can I get the most accurate ovulation dates?

I use this app and iP app and the dates never match, I enter my menstrual dates, but it never fails, the dates are always off.

I was trying the thermometer for about two months, but i started to confuse myself, wondering if my temp suppose to drop when I ovulate or increase or does it suppose to happen on my fertile moment, again drop or increase temp. Someone please point me in the right direction. We've been ttc for 4 months or so and I'm already frustrated. It's not as easy in my 40's as it was in my 20's. I get so down when my flow arrives. Please help.