The short answer is forget that as a method of rangefinding.

They are designed to work with a target of known size. In the early days of FT (Field Target) shooting the kill zones on the targets were the same size.

Given the above condition, you adjust the magnification on the scope such that the target just fits between the two horizontal lines and then simply read off the distance to target.

Some scopes even had the top scope turret similarly marked, you would twiddle the turret and then the scope would be correctly adjusted to allow you to aim directly at the target.

The more modern way is to use a laser rangefinder (LRF). If shooting competitions they aren't allowed.

In FT high magnification scopes are used, these have very shallow depth of field and hence this means the focus mechanism can be used to accurately estimate range. +/- 1 yard @ 55 yards is not impossible.

In HFT no adjustment to the scope is allowed once the round has begun, making any of the above redundant as means of estimating range. There are various techniques to assist in range estimation, but that's a whole different debate.

HTH