So I’m still kind of rebelling, I am sticking to my reference material and sizes. The screenshot shows the size I allocate for the bishops. I’m not that far off with my 38mm, I’m almost at 4BU as well. For scaling initially, I just shift-scaled until the diameter was right. The origin was set to the center of the bishop immediately (select bottom loop, snap cursor to selected, origin to cursor) and then this object can simply be placed at 0,0,0.
