HPM: How do you know that your pump and its gauge are accurate? Have you had them calibrated regularly and certified?
TUD: The gauge measures pressure. Pressure is defined as FORCE PER UNIT AREA, of the surface, with the area being measured at right angles to the direction of the force. Thus the pressure will be constant regardless of the orientation of the pencil gauge. However like HPM's gauge how accurate are your gauges in reality? That can only be determined by calibration.
I do not calibrate mine, but I only ever use the same pump and gauge to at least try and get consistency. HTH.