[shmeeed]

I'm sorry to say your conclusion doesn't hold. I'm not sure where exactly you went wrong, without trying to do the math my best guess would be that you just can't properly simulate a far-sight task in a near-sight environment like that. Our eyes don't work that way.

But if you go outside and do a real world test, you'll (hopefully) find that a number plate should be readable from much further away than 20 meters. If you don't, please go and see an optometrist! I'm serious.

20 m is actually a very lenient requirement IMO. In my country, one should be able to read a number plate from about 35 m easily, and with good eyesight, 65 m and more shouldn't be a problem.