http://www.sailing.org/tools/documents/ ... 20946].pdf
Racing Rules of sailing page 125.
300mm Minumum Height, Minimum 60mm Spacing between letters and edge of sail. Starboard High.
Except as provided in rules G1.3(d) and G1.3(e), class insignia,
national letters and sail numbers shall, if possible, be wholly
above an arc whose centre is the head point and whose radius
is 60% of the leech length. They shall be placed at different
heights on the two sides of the sail, those on the starboard side
being uppermost.