Nejimage Factor - Vol. 2 Ch. 10 - Error 10

[MangaDex]

 

[Cavans99]

I don't get if the problem here is me or how the translations were phrased.
The translation is even too good for this manga, but sometimes it's really hard to understand what some senteces mean. I've read page 7 ten times and it's still a mystery...

 

[Simpleton]

For the eyes, I suspect that Japan uses the Snellen scale, meaning 20 is "perfectly normal" and 18 is "slightly better than normal" (number higher than 20 means worse vision, but on a scale where the difference is greatest at low numbers, allowing for increasing granularity as visual acuity worsen)

Edit: This assumes the numbers are supposed to be 20 and 18 rather than 2.0 and 1.8, but if I had a nickel for every time someone has gotten a decimal point wrong when translating Japanese numbers, I'd be able to afford a cup of coffee.