I'm not an optician, but I do know that 20:20 vision means you can read letters 20mm high from 20 ft.
Given that its related to your ability to focus sufficiently to see detail at a specific distance, and that lenses work in a linear way, then I think it comes out as follows.
If a letter is 20mm high, it can be read from 20 ft away
So if the letters are 50cm high on the wing, that is 25 times bigger
By extension, you should be able to read them from 25x 20ft
So you should be able to read the letters on the underside of a wing at 500ft
Any guess as to whether that makes sense?
IPZ