How To Find The Exact Date Of Ovulation?

Hi I came off the pill in february and had a mild period this month and trying to work it out on my trackers it is showing me two different ovulation dates and plus i like a drink on a night and we have sex after a few days or within a few days quite quickly what do you advise as I am all over the place with my period and never know when i am ovulating, I am 35 years old, 5 foot 1 and weigh around 7 stone, i lost a bit of weight after comng off the pill thanks