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.
Mostly wrong - read wired's excellent explanation (e.g., here
Foot size is the primary determinant for proper board width, as it determines to what extent the rider can have proper edge control. Standing on the board with naked feet in the usual riding positions, the toes/heel should just be at the edge of the board or have slight overhang.
Size/dimensions of the boot will indeed determine toe and heel overhang/drag, but that is strictly a secondary concern.
You are correct that sidecut radius and stance width are important, as they affect width at the inserts - which I had already pointed out is the key dimension, rather than waist width.
But your generalization is not correct in my experience - 25.2cm WW should be enough for almost all 10.5 size boots almost regardless of sidecut. For instance, the GNU Billy Goat 159 has an even narrower waist and nose/tail than the board described by the OP and it still works just fine with 10.5 size boots.