CG has to be set IN THE AIR before you are done!
Most ARF manufacturers provide a "starting CG point" or a "CG range". Definitely, do try to get your new airplane close to this value on the bench, and it's usually better, if the maker provided a range, to be closer to the nose-heavy end of the range for a maiden if you are a less-experienced pilot.
However, realize that even among experienced builders, very few people can set CG correctly. There are many "CG machines" on the market, tools to help bulders set CG accurately...but despite this many people measure incorrectly or otherwise mess up in some way and get it wrong. It's a fact.
If you have assembled an ARF, and you find yourself adding lots of weight to the tail or nose to get your CG set, stop and look at what you are doing.
Is your motor the same size (weight) as recommended?
Is your battery pack(s) the same size as recommended?
Are you using the expected type and number of servos?
If you are doing the above and cannot get the CG set without massive modifications, you may be doing it wrong. Check your measurement, are you sure you are reading the ruler right? Some planes are sensitive to orientation, they need to be balanced either upright or inverted, make sure you are doing it the right way.
If you can find information about your plane on-line, check with other users. In my opinion, it's safer to copy the setup of a person who is successfully flying the plane than to go with your own CG measurement if it doesn't seem right.
A war story: We went to an event in '07 and two pilots showed up with identical 3DHs airplanes. Using the recommended setups and batteries as they both were, I knew from experience that a nice CG was easy to obtain with the lipoly pack in the center of the tray. One pilot had his pack up in the very nose of the plane, the other had his hanging off the back of the battery tray. Both, of course, "balanced" their planes at the same, recommended location.
So, to sum up, if it doesn't seem right, it probably isn't. Figure out why before proceeding!
So, once you have your CG set on the bench, it's time to fly. Setting CG in the air is a process of making your airplane fly right FOR YOU (not for anyone else). The following information is right for AEROBATIC AIRCRAFT. Flat-bottom wing aircraft like trainers should NOT be balanced with the method that follows.
Here's what you need to know:
Test your CG location by doing the "roll inverted" test. Fly your plane level at 2/3 or 3/4 throttle, and use the trim knobs on your radio to trim it fly *STRAIGHT AND LEVEL* by itself. Once it is perfectly trimmed, roll it over inverted. Observe what it does.
If it DIVES inverted, it is NOSE HEAVY.
If it KEEPS FLYING STRAIGHT inverted, it is NEUTRAL.
If it CLIMBS inverted, it is TAIL HEAVY.
Move your battery pack, or add/subtract nose/tail weight as needed to move your CG. Don't change the CG much, only about 1/8 inch at a time. Fly the airplane again with your adjustment. TRIM THE AIRPLANE AGAIN for straight and level flight, then do the roll-inverted test again. Repeat as necessary RE-TRIMMING the aircraft for STRAIGHT AND LEVEL EACH TIME before rolling inverted.
A VERY NOSE-HEAVY airplane, during the roll inverted test, will sharply dive while inverted. It will require a significant push down on the elevator stick to maintain altitude inverted. This CG is NOT good for aerobatics. It's OK for basic sport flying with many aircraft, but the airplane will not fly high-performance maneuvers well, and probably will not 3D much at all.
A SLIGHTLY NOSE HEAVY airplane, during the roll-inverted test, will gently descend when rolled inverted for the test. It will thus require a slight push down on the elevator stick to maintain altitude inverted.
What the vast majority of pilots want is a SLIGHTLY NOSE HEAVY airplane. This is where competition aircraft are usually balanced, this is where most sport aircraft are balanced. This location provides good tracking and easy landings. Most 3D aircraft will 3D well at this CG.
HOWEVER, the very best 3D performance will usually occur at a NEUTRAL CG location.
A NEUTRAL CG balanced aircraft will have lots of pitch (elevator) authority and will rotate around the wing spar very easily. It will often hover and harrier easier than a nose-heavy aircraft.
HOWEVER, a NEUTRAL CG balanced aircraft will not track very well. It is not going to easily fly precision figures. It will be difficult to land smoothly and will tend to balloon up on final approach.
A TAIL HEAVY airplane is only for experienced 3D pilots. It will NOT fly precisely at high speed. It will be very challenging to land smoothly. It may have very high 3D performance at low speeds, but be cautious with any setup balanced like this until you are experienced with it.
Experiment with different CG locations. Move your CG only a bit (1/8 inch or so) at a time, so you are not taken by surprise when making changes, but do experiment so that YOU will learn how different CG points feel and what YOU like to fly. Remember, this airplane is YOURS and it should feel right to YOU, not your flying buddies or anyone else!