I hear that the standard for CG is between a quarter and a third of the cord from the leading edge. Anything from this overall guideline should be considered "too far"?

Now on this nose-heavy, tail-heavy thing. As gus explained this nose-heavy and tail-heavy is relative to the intended CG. Lift your plane at the intended CG. If it is balanced, it means that the plane should fly as the designer had intended it to. If the nose goes down, then it is nose heavy (relative to the original design), which means that it will fly with a nose down attitude relative to the original design. Conversely, if the nose goes up, the plane is tail-heavy and will fly with a nose up attitude relative to the original design. That is it. Finished, end of discussion.

I'm going to put something down for those that are interested and knowledgeable to comment on the following:

1. Each plane is designed with a certain distance between the CG and a point I will refer to as the Center of Lift, which is the point where at a neutral attitude (zero or predetermined angle of attack) the average of lift is. Depending on the nature of the plane the position of the CG relative to the Center of Lift will change:

Trainer - CG will be forward the center of lift so that forward momentum via gravity is maintained
Pattern plane - CG should be near or at the center of lift for more neutral handling
Aerobatic plane - CG may be at or behind the center of lift so that the plane is twitchy and extremely sensitive to inputs and must be flown

2. Nose heavy planes are those planes with CG forward the Center of Lift whereby the plane is always wanting to go forward. In a deadstick these planes will not require the pilot to be too worried about stalling.

3. Tail heavy planes are those planes with CG behind the Center of Lift whereby the plane is inherently unstable. In event of a deadstick these planes needs to be flown all the way or else they will stall.

4. This also explains why high wing is the trainer of choice. Not only does the high wing offer a CG that is below and forward the Center of Lift so that it maintains upright. Imagine the Center of Lift is the pivot and the CG is the weight. In a dive, the CG will want to swing the plane back so that it is relatively below the Center of Lift. Same thing in a climb. Thus stalls are more difficult, they have to be induced. Now look at a low wing acrobatic plane where the CG is above and on top of the Center of Lift. It's like trying to balance a wrench on your fingertip. In a dive, the CG will want to push the plane further into a dive. In a climb the plane will want to pull it further into a stall.

Anyway... just my thoughts that were inspired by this CG discussion.