Assuming to have had just about as much experience as anyone else on here well versed with Zegna pieces (for personal and professional reasons) other than Brain most likely and of course Spoo, I believe I can be of some help here. 1. With Su Misura pieces, disregard all posted measurements, except the length, this is 99% correct, albeit fairly hard to fuck up. I actually have a Cash Su Misura tagged almost identical, 62R, chest is like a 44 or something, the Shoulders are quite wide though, however regardless. 2. With Zegna, the market, buyers and sellers, know it as the measurements not aligning with a classically measured suit i.e. classic inches measured to tagged size ratio, for example, 42R usually measure 21.5-21.75, not spot on but on average below most industry standards. I've found when stuck with a Zegna piece that actually measures to an exact, list it as that exact (in this case, simply 48L) because this results in higher traffic. However be careful when doing this, because there are bullshit technicalities that can be claimed by a buyer with remorse or whatever (if you accept returns, I don't, but just in case). Make sure the tagged size is documented, then the measured US size is given as the equivalent. Just don't leave yourself open for assholes.
+1 "With Su Misura pieces, disregard all posted measurements"