I took a long route on sizing for the Y2 Chore as I'm in between an IT48 (Belstaff), US38/Small or US40/Med (Schott), Small (Allsaints), and typically 40 (Y2 and DoubleHelix). I burned shipping $ and ordered both a 40 and 42 in the suede versions with the idea of returning the size that didn't work. I tried them on w/ single layers, light sweater, and a flannels). I will say, this particular cut that TJ designed is forgiving (i.e. you can go up or down a bit w/ reasonable margin of error).
Ultimately I stuck with the size 40 for the chore (across the board for my Y2s) based on this trial/error along with some big forum help here from @Anesthetist and others on comparable fit to other styles (it looked a bit more fit/polished on my frame IMHO). I have plenty of room to move, and a bit of room to layer, but 9 / 10 if I'm wearing this I'm not layering too heavy or I'm wearing an heavier coat on top. For reference I'm a IH Large in shirt sizes.
Hope this helps.