the follow back rate can't be measured by looking at how many you follow a day and how many you gain a day.
to have the concrete number of follow backs you have to follow for example 7,5k people, then unfollow those not follow you back ONLY to get the real number of follow backs.
to be a bit more specific:
follow back % = (total follows - users not following back)/(total follows) * 100 [%]
Many claim getting a follow back of 50-60%, which is very hard, unless you invested quite some time to be super targeted.
Remember that you can also gain followers from your likes and comments, although commenting limits are really strict on IG, or even your post being featured of many people's explore feed.
Many here claiming having a follow back of 60-70% have it maybe at 40-50% max. keep that in mind.