A dressing mirror hung at the wrong height is a mirror people stop using, so we ask a question that most mirror makers do not: how tall is the person who will use it?
The useful part of a mirror this size is a band around its middle. Hang it so that band sits too high and you are looking at your hairline; too low and you are stooping. Neither is dramatic, and that is exactly why it goes unfixed for years - it is a small daily annoyance rather than an obvious fault.
So the hanging position is worked out before the mirror leaves us. From the height you give us, we calculate where the centre of the plate should sit, and from that where the fixing needs to go on the wall - accounting for the drop between the hanging point on the back rail and the centre of the glass, which is not the same distance on every design.
The back rail is then drilled at two points rather than one, a little apart vertically, so the mirror can be moved up or down after it is first offered to the wall.
The measurement is written on the paperwork that travels with the mirror.