There's some discussion in this thread, buried in all the razzing (and the actual build photos): http://stunthanger.com/smf/index.php?topic=27915.0

Basically, you want it to be far enough back so that when the tank is empty (or the battery discharged, of course) you can push the tail down until it touches the ground and have the plane fall back level.  I've also heard that you want it about 15 degrees back from the CG (which is about the same as the "won't stay tipped over" rule).

If it's too far forward then the nose won't go down positively on landing, and it may "stick" up when you're done landing.  If it's too far back then the plane will not take off or land smoothly.

(My trike gear Twister lands very nicely, BTW, except when I mess up).