Gabe, the instruction sheet isn't worded very well. The 1/2" measurement is from the top of the shim pack to the bottom of the plastic shoe. When they say top of the guide, what they are talking about is the surface where the bottom of the spring rests. In other words, if you look at the guide from the side it looks like a "U". The surface they are talking about is at the bottom of that "U". Click on the attachment to expand it to full size and look at the red arrows I added.
If Kathy from Hayden sees this thread, perhaps she could convince someone at Hayden to fix those instructions and actually show on the picture where the measurement should be taken.
Jerry