Actually you're just changing where the plane wants to natural balance. Which is why you need to setup Duel Rates for either forward CG or rear. If you have low, medium and high on the radio that's an easy setup.
As I remember it. The setup used little springs on each end of the high strength Nylon string. As metal cable cuts into the spool over time. The servo was mounted side ways so the spool was on one side and the string as far left or right as lose to the fuselage just so no stray servo cables could wrap around it. You wrap the string around the spool a few times then the springs cause tension. The tray had 2 sets of tubes. You use the inner tube cut to length then threaded rods on each end secured to thin sheets of ply on each end. Then use nuts on the threaded ends with washers to support it being sure to install the outside tube first. Glue the outside tube to the tray and test with batteries and notice where the CG moves.
With todays radios you can use the dial knob for forwards and back and set the servo travel. That way you know where center, forward and aft are all the time.
Now this was in the early 80's and the guy was a pattern flyer. He was so anal about his planes. I once seen him cut the whole rear stab and elevator section off just forward of them with a band saw. He said it was too nose heavy and off center. So he made it longer. As this guy said adding weights to an airframe to make it balance was for imbeciles that had no idea about aerodynamics and adding rocks to fix it means they didn't build it right. I didn't argue with him. He out weighed me by 60 lbs!