CG location is in part a matter of taste, nevertheless here is an easy way to know if we are in the right path or not. This test allows you to disregard the motor incidence and the elevator trim and focus only in CG location.
The test has 3 easy phases, and if you are flying glow it should be performed with the tank almost empty (worse case)
Step 1: put the model at a safe altitude and adjust throttle and elevator trim to fly straight and levered without any extra input. It can take few minutes to achieve this, but it is important for the results.
Step2: push the elevator stick and make the model descent in a 30° line until you notice the model acquired a higher speed. Do not touch throttle.
Step 3: release both sticks and see what happen.
Finally, to end the test just pull the elevator stick and add some throttle to gain altitude again.
Five things that can happen:
1) the model goes to a nose up attitude and start to go up and down in a pendulum movement never reaching the equilibrium. It means that the CG is very fore and should be set back. This situation is not comfortable and can be dangerous.
2) the model goes to a nose up attitude, but after one or two tries it recover the straight and levered flight. It means that the CG is aft, a good situation if you are flying a trainer, but not so good for an aerobatic model.
3) the model smoothly recover the levered flight. This condition means a perfect location of the CG for most of the sport and basic acrobatic models.
4) the model continuous in the same descending line that it has after releasing the stick. This condition is the ideal for advanced pilots flying aerobatic and 3D models.
5) The model sinks even more deeply. This means that the CG is too aft. This situation is dangerous and do not represent any advantage for the model performance.
Always try all this at a safe altitude and in a calm day, repeat the test until you get consistent results, and try to the get the help of a buddy to look and discuss results.
Every change in the CG must be small and documented. Be aware that, at after some point, a little further change will have a huge effect in the model’s stability, so be prudent. A CG located at 25% of the chord is a safe starting point. In the other hand is hard to find a CG located beyond the 35% of the chord.