Procedure for Estimating Distance with a Sight Level
At first, you need to look through the device. Then we need to note down the top and bottom lines of the viewing window. Generally, the ratio of bottom and top is 1:12.
Here is an example, if a user looks through sight level of the instrument and sees the top line at six fleet's of the rod and bottom line at three fleets, so the user is 36 feet away from the rod.
First, we are going to find the difference between the two distances in the viewing window; it's just 6 - 3 = 3. After that, we calculate the difference and multiply by 12.
It's done because to convert from thee distance ratio to the actual distance, that is 3 x 12 = 36.