Just to contribute to what Bustr said...
An additional 10 feet in either direction from the center of the circle means each circle grows in diameter by 20 feet. This means the inner circle is 20 feet in diameter. The second ring is 40 feet in diameter.
Most fighter aircraft (with wingspans typically measuring between 30-40 feet across) only fill the inner one-and-a-half to two circles. This is useful, as it allows you to examine your bullet dispersion pattern on the target and imagine where or if that would be hitting various aircraft.
Another good thing to do is bring up the target at your set convergence and compare that to your gunsight reticle. This will allow you to see what a fighter wingspan looks like at your set convergence. For example, if your convergence is 350 yards, set the target there (with the command .target 350). Then fly level with your pipper on the center of the target. Note how much of your gunsight reticle is filled by the inner one-and-a-half to two circles. That's the average wingspan of a fighter when they are at your firing convergence.
Hope this extra info helps!
<S>
KP
p.s. The .target command works in all arenas, so don't forget to turn it off when you are done (.target 0), or it may come up on your screen when you change arenas and launch in the MA.