These Yang Li derbies are really nice.
What fall collections are you guys looking forward to?
In European sizing, your chest measurement is indeed your jacket size. In American sizing, it's basically 10 more. I.e., you're a 40 in EU sizes, and a 50 in US sizes.
^ If you measure 18" shoulders and 40" chest, you're likely a size 40 in suit jackets (or a 50). That's pretty standard/normal.
I wouldn't say that translates to 1 to 1 for TOJ though...I would use their actual measurements listed for each size.
To answer above question, eu 44
And to answer what size you are, I'd guess you're a large eu48
Lastly, unless we're actually talking about a suit-jacket, shoulder measurements honestly aren't that important.
Like, 0,5"+/- on a unstructured jacket won't matter much as long as everything else fits well