Irregular cycles? How do I pinpoint ovulation?

Last three cycles have been 51 days, 49 days, and this last one was 34 days. Before that I had no cycles for for almost two years from breastfeeding. I have this app and Ovia which both tell me different fertile weeks and I am confused. Don't want to waste money on two packs of opks. How do I pinpoint ovulation? Or should I just suck it up and buy more opks?