Roelof is correct. This tool only checks the rotor's magnetic strength.

When shimming, there are a few ideas on what is the best way to shim a motor.

• Shim the rotor to be as close to the sensors as possible so the readings are more accurate.
• Shim the rotor so it is centered in the middle of the stator. The theory is, this will give it more power.

Whichever way you choose, the main reason for shimming is to allow as little side-to-side slope as possible, with your rotor. Just make sure you don't shim the rotor to be too tight.