There is not a set anwer to this. As long as you can bend your knees to a 90 degree or better ( I like to be able to drop my ass to the ground) without feeling a pull on the out side of the knee caps, the width is fine. As for angles, it depends on what you are doing. Your shoulders referance alignment will be the same as your front foot. I.e. if you have your front binding set at a 15 back binding 0, then your shoulders will open up to a 15 towards the fall line. You want to try and keep the split (or difference between the angles) as close to zero as possible, so your shoulders stay with the board. The wider the stance, the lower your center of gravity, which equals more stability, and forces you to keep equal pressure on both feet. As opposed to riding only on your back leg.
My freestyle/jib/pipe board I ride 15/-15, my free ride/everything board 8/-8 but the same 20 1/2 instep on both set ups.