There is no "right spot" in full suspension, put the rope around your neck, kick the chair it's over. The knot can be at the front or at the back of your neck, doesn't really matter. That's why you don't see forum posts like "I've tried full suspension but couldn't pass out" xD
The "right spot" is only a thing in partial suspension because in sitting/kneeling position you're not putting that much weight on the rope, so it's harder to compress the arteries.