I use a little different system. I remove the prop and hook a loop in a suspended string over the prop shaft. I slip another string under the rudder hinge closest the thrustline of the engine. I do it once just before covering with all systems in place and again after covering. Note that if you haven't glued the hinges in you should masking tape the rudder to the vert. stab.

One of my favorite balancing methods is to place pennies over the aileron servo on the high wing. Then I remove the servo and tape the pennies to the servo housing with black electrical tape. Cheap and easy and you can alter it after the covering is on.