Another way to do the same thing is length / two, and then either add or subtract to move it forward or rearward…
5’6" = 66 inches. 66" / 2 = 33" - which will be your midpoint. When measuring from the tail, anything in excess of 33" would be forward of center and anything less than 33" would be behind center. Reversed, if measuring from the nose, of course.