don't know if this is what Brett uses, but the standard tail volume calculation is:
tail moment * tail area/avg chord*wing area
tail moment is the distance from the CG to the MAC of the stab, but since this is kind of an estimate(it changes when the plane is maneuvering) you can use the distance from the wing MAC to the stab MAC and still be close enough.
the units cancel out and you get a number which is roughly indicative of the effectiveness of the tail- bigger is more stable, smaller is less stable. Typical numbers range from .1 for a combat plane to around .5 for many modern stunters and scale planes.