The ESR is measured by dividing the IPD (interpupilary distance) from the bizygomatic width (zygo to zygo).
The ideal ratio is in the range of 0.46-48.
How to measure it?
There are two major methods to measure that ratio and any other facial ratio.
The most accurate one is to measure using a cephalometer or caliper.
With this method you can calculate the distance in mm accurately unaffected by lens distortion, thus reducing measuring errors.
Alternatively just take a picture of yourself and take a few steps back from the camera to reduce lens distortion.
After that you can either measure the pixels with photoshop or go on a measurement website.
You can use this website for example:
Keep in mind the AI isn't accurate, so most of the time you have to readjust the points by moving them.
Here a few examples of the ideal ESR:
Just keep in mind ratios are for the most part just mental masturbation. Everyone adds or subtracts a few mm's to get to the perfect ratio.
There is not much you can do about that ratio besides blackpilled surgeries like OBO with tripods and infraorbital-malar implants.