How are ROSS blanks measured? ROSS key blanks are measured from the head to the flag. If the door thickness is known you can determine the key blank required using the table below. ROSS blank Door thickness R30 up to 20 mm R40 up to 30 mm R50 up to 40 mm R75 up to 65 mm R100 up to 90 mm R150 up to 140 mm The rule is typically the blank measure minus 10 mm. E.g. A 30 mm ROSS blank will suit a door thickness up to 20 mm.