Abstract: A guide bracket is attached to a screw gun that is used to drive screws into a window rough opening. The heads of the screws are used to define set offs to position a window jamb box in a desired position in the window opening relative to a window installed in the opening. The bracket is attached to the screw gun in any appropriate manner and includes a stop member that stops travel of the screw gun as it is being operated to drive the set off screws; the stop abuts a reference surface on the window frame to thereby fix the position of the jamb box when installed into the rough opening.