I had an idea just now, while considering this thread. If ordering a set of prescription shooting glasses, and wanting the lenses ground to your Rx in the right spot, why not mount your shotgun and have a helper snap a pic of you from the muzzle(s)? Forwarding that pic to the company would show them exactly where you are looking through the lens, and should be very helpful to them in tailoring the glasses to you. Should it not?

May God bless America and those who defend her.