Tail volume is a formula that gives a value that'll suggest how effective the tail will be. It's actually pretty easy to do the math.

You need to work out or measure 4 things:
Wing chord (actually the MAC)
Wing area
H.Tail area
Tail arm

The wing measurements are talked about all the time. You're interested in the mean aerodynamic chord. Wing area is easy, right. You'll also need to mark on your plans where .25MAC is for the wing. You'll need that to work out the tail arm.

The H.Tail area is the same. And you'll need the .25MAC point for the h.tail too.

The distance between the .25MAC of the wing and the .25MAC of the h.tail is the tail arm. There is more than one way to base this measurement, but the 25% deal works for the overall formula just fine.

TAIL VOLUME = (H.TAIL AREA / WING AREA) X (TAIL ARM / WING CHORD)