advice on ovulation & pre seed

According to glow I'm due to ovulate between the 12th & 18th but according to Ovia I'm supposed to ovulate between 7th & 12th. I've been using opk's but last month when I used them I was confused because both lines were always dark. I bought Pre Seed to try this month but don't wanna waste it. How many uses can you typically get from it? & is there any other way to help figure out when I ovulate for sure? I don't wanna do bbt because I haven't done it before & I wouldn't understand it right away. Just want this to be our month.