fiat 850 gearbox special tool

[fiat850]

Hi, does anyone know what this special tool is used to actually measure?
I know it is to determine the thickness of the shim between the pinion and bearing, but I don't know how this works and if there is another way to determine the thickness other than by trial and error
Thanks

[Sabookle]

I'm ready to do everything, so everything is going well.