Usually, the arc flash boundary would be the distance to the closest energized parts. This is one reason why the working distance, the distance that the energy is calculated from under normal conditions, may be different. For instance, in Medium Voltage switchgear, most of the energized parts are deeper in the cubicles than in a 480V panelboard.
That being said, I would suggest measuring your arc flash boundary from the outside of the panel or switchgear. This would result in a conservative boundary, and also prevent you from having to stick your tape measure up to the bus work.
