Sorry, Jim, it's hard to explain without a diagram. Conceptually, the tail arm is the average distance between the wing and the stab. The simple way is to measure from the leading edge of the wing to the leading edge of the stab.

For a straight, constant chord wing, of course, it is easy. But the stab is almost always swept back some -- just take about the average point, but a little closer to the fuse. The concept is to measure at the point on the leading edge where half the area is on one side and half is on the other (the 'MAC'). You can calculate it, but I always eyeball it. Same with the wing, if it has taper or sweepback.

This formula is in many books. Gordon Whitehead, in his book on scale models, has a different version that is algebraically the same.

Gordon points out a nice use for the formula. Say you want to build a WWI model that is notorious for flipping on its nose when it lands. Using the formula, you can calculate how much to enlarge the stab in order to move the CG back, say, a half inch. It is much less noticeable than angling the gear forward.

If the wing and/or stab are elliptical, what then? I take a piece of cardboard, measure it, and weigh it. Then I trace around the wing or stab outline, cut off the rest and weigh again. The area is proportional to the weight.

Jim