Aircraft Finishes If registration numbers are to be applied to an aircraft with a letter height of 12 inches, what is the minimum space required for the registration mark N1683C? Note: 2/3 x height = character width. 1/6 x height = width for 1. 1/4 x 2/3 height = spacing. 1/6 x height = stroke or line width. a. 52 inches. b. 48 inches. c. 57 inches.