The right side of the distance box corresponds to the side where the meter is physically closest to the projector. This ensure that the meter gets the most light for the most accurate readings, without getting into the saturation region where the readings would no longer be correct.I see also why the suggestion to start the calibration with the meter positioned towards the front/right of the distance box. As you start the calibration with the Iris fully open, and progressively calibrate at less open settings through the range, the effective meter position moves away rom the meter with each, more constricted, iris setting. The physical position of the meter doesn't move, but because there is less light entering the meter it thinks it's further away. Starting with the open iris in the front of the box means all other iris settings will also lie within the box without having to change the meter physical meter position.
Not sure if others do it, but I physically move the meter closer as the iris gets closed down, to keep it near the right side of the distance box.