TTC with an irregular period

I'm trying to understand how this app works with some one who has an irregular cycles. My MC can range any where from 28days to 45+ days. But the app goes by a specific day range. How do I go about keeping track of everything when I'm not sure when my cycle will end every month? My bbt fluctuates so much, I can't get an accurate read to know when I ovulate. I have a 3 year old but she was a surprise. It's all getting really frustrating.