Correct way to measure for glass eyes?

I have seven resin 1.1 busts that I am ready to start painting and put glass eyes in.

What is the proper way to measure for glass eyes?

Do I take my calipers and go from corner to corner of the eye?

So say my measurement is 28mm corner to corner.... I then order a 28mm eye?

I just don't want to order wrong and the glass eyes be to small/big and not be able to return them.

Thanks for any help.
Whenever I measure for glass eyes I usually measure from corner to corner of the eye and then add on 4mm (2mm extra for either side). Doing this has served me well in the past. But if there's anyone with better ideas, I'm all ears too.
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