My dealer told me the belt is too loose if you are able to twist it 90 degrees when it is in place on the car...
Agree that it's hard to judge 10mm of deflection...
According to the PDI check list point 31 a 1/4 turn is correct.
To measure 10mm I would think it would be possible to use a wooden dowel or thin baton held up to the belt & against some fixed point on the car or ground, mark where the belt sits & a second mark 10mm apart & then check the deflection. I have not tried this
