Now it was time to set our baseline backspacing dimension. The vertical extensions of the tool will slide forward and backwards on the tool to simulate different backspacing measurements. We simply loosened the lock nuts and moved the extensions until the back edge of the tool hit the number we were after. We started at 5.5 inches of backspacing, as indicated by the numbers engraved on the horizontal extensions, which lined up with the back edge of the vertical extension. This part was important: the faceplate of the tool is 1/2-inch thick, so we had to subtract that number from the final backspacing measurement to know the true measurement of the wheels.