As the flag measures 4 ft. by 3 ft., the length of the diagonal (from
corner to corner) is 5 ft. All you need do is to deduct half the
length of this diagonal (2½ ft.) from a quarter of the distance all
round the edge of the flag (3½ ft.)--a quarter of 14 ft. The
difference (1 ft.) is the required width of the arm of the red cross.
The area of the cross will then be the same as that of the white
ground.