To check the deflection with the tool, slide the O-ring to the 10-pound line, put the tool perpendicular to the belt midway between the front and rear pulley, and note the measurement with zero pressure on the belt. Harley does you the favor of putting a window and 1/8th-inch graduation marks on the belt guard, but the tool has a ruler scribed on it as well. Once you’ve got your starting reference point, press on the bottom of the tool until the O-ring just touches the blue anodized based and measure the deflection. It’s a good idea to rotate the wheel and check the belt in several locations and then average the figure. That means adding fractions, but you’ll have to look up a separate how-to video for tips on how to do that.