You have the same problem as me, and we are probably my size but you probably have \ broader shoulders. I'm also a true 40 chest , possibly a little bigger, and I take a size 3 in Blue Blue Japan. My waist is bigger - um, I do have a pretty strong core from years of MMA, but that's not that reason I am wearing a size 33 jean right now. If you have wider shoulder for a size 40, you are going to want to go up a size 4 for the fitted jackets. I have a chore coat in a size 3, and have had to restitch the shoulder seams a couple of times, because the thing was made for someone with the same chest size, but with much smaller back muscles. The oversized coats , on the other hand, like the sashiko long coat from last winter, for example, are truly oversized, including in the arms and shoulders. I've worn that over a pretty substantial leather jacket with room to spare. Of course, wearing those coats fitted (Mr Porter suggested sizing the same as your US size) will throw off all the proportions.