help track ovulation

Lea

ive had this app for alkost a year 2 track periods an ovulation. last 6 months been irratic an i dont fully truat this app considering my cycle days vary so much. last 6 months are. aug.35 day cycle sept 39. oct 42. nov 42 dec 26 jan 35. i am not on any birth control or medications either... whats a better way 2 track my fertile window considering my cycles vary over a week appart?! thankx ladies