DIY: a quick outline on why & how to set pinion angle.

[Ivan141]

The gauge probably works with an accelerometer, which references ainst gravity. You could also use a smartphone with an app to do this.
Since your measuring a vertical surface, you'll probably need to zero the measurement against something perpendicular to the floor the car is sat on.
That measuring cube has the advantage over a phone in that regard, you can just zero it on the floor.