I've used the plumb bob method on my two jets and both flew with no worries (Yellow F-4 and Byron F-15 twin)

Check out Joe Luptons site for full details and Stuka pic (above) gives you great info. http://members.cox.net/jluptonjets/

If gear retracts aft I balance with gear up. Cycling gear while hanging gives you idea of how much shift in CG also (usually 1/8-1/4 inch) so you are prepared. Once that is done I add fuel to see if CG shifts to far aft.
- (For F-4, to help rotation with scale gear location on takeoff, Bob Fiorenze, recommended changing fuel tank location aft and fan forward. He was right!) CG when fully fueled and gear up is 1/8 aft of panel line at (8/14 CG) and then burns down during flight for landings.

I'll have to try Darryl's method also as it seams very simple.