What the hell does foot size have to do with board width? You're determining heel and toe drag. That's dependent on the outside profile of the boot, which can vary by up to a full size depending on the model. Then you'll have to figure in sidecut radius and stance width along that radius. You're dealing in terms of millimeters here. So choosing a waist width is really just a general rule, and by my experience 25.2 is too narrow, generally, for a 10.5 boot, unless you're riding boiler plate.