The bike needs to be oriented upright (off the side stand) with no load on the saddles. This pivots the swingarm close to the position of maximum belt tension (where the rear axle, front sprocket C/L, and swingarm pivot are all aligned). Measure belt deflection with the gauge using 10 lbs of force at the center of the free length of belt. The belt should deflect 8 mm on a VRSCD (I'm assuming you have a nightrod, the spec. varies with year and model).