That chamfer trick didn't work for me at all. I just did a room polyline moulding and drew the moulding to attach to it. Had to mess with the moulding settings a bit but visually it's perfect. As far as materials quantity, since it overlaps the floor slab I figure the quantity will be about right. The only thing missing is the quantity of re-bar, but it's also not tracking the WWM in the floor, so when I export the materials .csv, I just add those two line items in Excel. I figure nobody orders 117'3-1/2" of rebar anyway.....
