This "tail in the box" rule is also known as "grandma's rule". Whatever you call it, it's bogus. It will produce a badly undersized V-tail.

For simple sizing V-tails, see http://www.charlesriverrc.org/articl...tailsizing.htm
The DJ Aerotech site has similar stuff.

The most detailed V-tail info I know of is here:
http://www.fmsg-alling.de/vtail.htm
but it's probabably more than most people want to deal with.