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there is probably no correct way to answer your question without additional information from your side...
Are we talking about A-2s or B-3s? Are you referring to his civilian style jackets?
As far as A-2 jackets are concerned, most of us here have made the experience that Shawn's sizing tables are pretty accurate. I can say that for my Doniger and my Dubow...
I would need a size 48 from other makers, and my FS jackets are a 48 (Doniger) and a 50 (Dubow). I would think that the 50 is owed to the contract specifics of the Dubow, not the fact that the FS Dubow runs unusually small.
His B-3 jackets seem to have long-ish sleeves...compared to most other repro makers.