Agile-Shop Universal Magnetic Gauge Tool For Car/Truck Camber Castor Strut Wheel Alignment

How can I measure my caster angle with this? It appears to be only a camber gauge.

How effective is for alignment

This is a camber only gauge is it not? How are you to adjust castor? Which way do you turn the wheels 20° to first? Different gauges require their own

What is the exact procedure for measuring caster with this tool?