It does not need to be compared to a track. The tool is used to compare cars and their downforce. As long as all of us are testing to the same scale, then all cars are balanced vs each other.
Obviously I understand that, my point being 150g reading on the tester won't equate to 150g on the track.It's a measuring tool so you can compare one car against another. I have a Marshall Magnet and it works great as a set up tool as well as a tech tool. Same as the tool pictured above.
Bob