Or there's an equal chance for them to be bots, which either follow back or no.
If you aim at getting the most possible followbacks, i think it's better, if you target followback hashtags.
The ratio calculation is pretty easy, basic algebra.
If you have 1 follower and follow only 1 person, the ratio is 1:1=1.
For 1,000 followers and 1,000 followings the ratio is 1,000:1,000=1.
If you have more followers than followings, your ratio is higher than 1:
For 1,000 followers and 500 followings the ratio is 1,000:500=2. You have 2 followers for every single following.
If you have less followers than followings, you ratio is less than 1:
For 500 followers and 1,000 followings the ratio is 500:1,000=0.5. You have 0.5 followers for every single following or 1 follower for every 2 followings.