The nut is a locking nut. Turning this wont alter the boost. You undo the locking nut, and then u have to take the actuator bar off the turbo. Lenghthen it to reduce the boost. It cant be too long though - u need to have at least half a hole preload - ie u need to pull the rod out of the actuator by at least half the diamater of the hole which goes onto the turbo - otherwise it wont boost properly.