Allan,
If you are asking about the Tail Volume Coefficient, as calculated by the following formula:
Tail Volume Coefficient
Tail Volume = (Tail Area/Wing Area) X ( Tail Arm/Wing Avg. Chord) where:
Tail Area = area of the horizontal stabilizer
Wing Area = area of the wing
[both areas include that encased by/covered by the fuselage]
Tail Arm = distance from LE of wing to LE of stab [for untapered surfaces; for tapered, use LEs at average chord]
Wing Avg. Chord = area / wing span
Billy's airplane TVo = 0.24131
The profile airplane TVo = 0.22465
Pat