I have one of those, and use it to set the rear width and make sure the rear tires are centered on the pod.

1)Add shims to rear axle until width reads 172mm (I use a set of 12" Harbor Freight Digital calipers to check this.) Doesn't matter what side you put the shims on at this point.

2)Put the metal block up against the rear wheel as shown in the picture. Leave the thumbscrew that holds the rod loose.

3)Push on the rod until it barely touches the side of the chassis as shown in the picture. Tighten the thumbscrew.

4) Put the gauge on the other rear wheel and sweep the rod so it passes the side of the chassis. The rod should barely touch the side of the chassis. If the rod doesn't touch the chassis or hits with the side of the rod, the axle isn't centered. Move the shims on the rear axle, reset the rod and try again. When the rod barely touches the side of the chassis on both sides, the axle is centered.

It takes a few tries to get it right, but once you do you can note how many shims are on each side and you don't have to mess with it again unless you change rear wheel brands (different offset).

Hope this helps. I saw you post this on rcracing.com so i'm guessing you're local to the NorCal area. Next time you come to Norcal Hobbies hit me up and I can show you how to use it.

-James