Thanks! What about sites (such as Warby Parker) that make you hold up a credit card (which has a standard size) and use that to determine (via what I assume is

Thanks! What about sites (such as Warby Parker) that make you hold up a credit card (which has a standard size) and use that to determine (via what I assume is a rule of three) the PD? [0] I can see of course problems with the card not being “perpendicular” and other 3d distortions. [0] - https://www.warbyparker.com/pd/instructions https://www.warbyparker.com/pd/instructions