You will need a transparent (plastic) ruler and a phone with cameras front and back.
1. Turn on the phone camera and use the front one so you can see your face.
2. Position ruler on the root of your nose so that the numbers are just above the eye.
3. Now comes the most important step. Try to position the ruler in the straight line, parallel with your eye level so the ruler is at equal distance from your pupils, iris, then take a photo. This can be difficult to achieve, however there is a trick. Move the ruler successively just a fraction of degree up and down around the imagined straight, parallel line and take a picture for each position. These are small changes in ruler's elevation. Make at least 10 pictures.
5. Send pictures from your phone to your email and open them in a picture viewer. I use Irfan View. If you downloaded picture all in one folder you will be able to view them in Irfan View one picture after another and you will clearly see which ruler's position and elevation is the most accurate. After you select that picture simply read the numbers (length) on the ruler between the centers of your pupils. Accuracy can be as good as 0.5mm although you will be ok with 1mm precision.
I used this method to order glasses from EyeBuyDirect, but any company that offers frames and prescription glasses can use your precision measurement of the distance between your pupils.
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