Print at 100%. Do not fit to page.
Then measure it. The two marker centres must be exactly 160 mm apart. If they are not, the print scaled and every measurement taken against this card will be wrong.

Lay the card flat beside the tray, in the same light and the same frame, for every photograph. Do not let it fall into shadow. Replace it when it gets dirty or creased — a stained white patch shifts every reading after it.