Place the rear binding on the chosen location, and screw it down loose enough so that the base plate can be rotated. Step into the binding and, with hips again facing the angle of the front foot, find an angle of the rear foot that feels solid and comfortable. Lock the binding down at this angle.
The splay angle of the feet is usually a result of collapse of the feet to the medial (arch) side. This type of collapse is the statistical norm. At a fixed stance width, it is necessary to splay the feet so that the metatarsal heads of both feet can contact the ground. This allows for a more favorable load bearing structure. In an ideal world, feet would not collapse this way, and a rider could stand with feet closer to parallel. This would simplify the sequence of movements used to ride. If the feet are properly supported by a quality footbed, the splay angle can be reduced.
If a board is set up without supported feet, and/or with a pair of boots that are not sized properly, it will be necessary to revisit the setup procedure. Changes will not be drastic, yet may be significant in their outcome.