4. Now it's time to set our baseline backspacing dimension. The vertical extensions of the tool will slide forward and backward 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 5-1/2 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 is important: The faceplate of the tool is 1/2-inch thick, so you must subtract that number from your final backspacing measurement to know the true measurement of your wheels.