Originally posted by Slice Of Light ...If the shim is thinner than the correct value, you get back focus, if the shim is thicker than you get front focus
And make sure that the diopter adjustment is set correctly for your eyesight - the screen markings should appear in focus without having to strain whichever eye you normally use.
Remember that shimming the screen is nothing to do with the camera autofocus or its adjustment, what you are trying to achieve is to have the image on the focus screen focused when the image on the sensor is focused, which should be when the autofocus thinks that it is focused