It will depend both on focal length, and the distance to your subject. If you are doing landscape, focusing at infinity, then when everything is in focus your lens will be about 5 1/2 " from your ground glass, so pick the hole which would allow you to turn the focus knob and adjust either side of this. If focusing on something closer, the lens will be further away, and you will want to use one of the holes that is further from the ground glass.