1) MEASURE the CG on your plane. Do whatever is necessary to adjust it to 31mm, which is the factory recommended starting point. Do this before you try anything else.
2) Zero your tx trims. If your transmitter allows, adjust the trim-steps to the smallest setting. (Usually "1".)
3) Power-up the plane, but do not engage AS3X. Then disconnect the battery.
4) Mechanically trim the elevator so that it's neutral with respect to the stab.
5) Take the plane up for a flight with the CG @ 31mm & see how it behaves power-off & power-on.
6) If it glides smoothly & doesn't balloon under power, you're done.
7) If it porpoises during the glide and/or balloons under power - add down-trim until it glides smoothly, then zero your tx trim & transfer the tx trim to mechanical trim.
8) Note the degrees of down-trim with respect to the stab. If it's only a few degrees or so, you can call it good - unless you're trying to squeeze every last bit of glide performance out of the airframe that you can get. If you're trying to set a personal UMXR glide record, adjust the decalage accordingly, so that no visible trim is needed for best glide with the CG at 31-32mm. If the plane needs a lot of visible down-trim with the CG at 31-32mm, the decalage is off enough to warrant fixing it or returning the plane & getting a replacement.
Remember that you must re-trim the elevator for best glide whenever you change the CG. That is absolutely mandatory. If you don't, you will simply end up chasing your tail & get frustrated with the process of flight-trimming. When you get it right, you will be rewarded with a plane that glides smoothly & efficiently, and exhibits very little (if any) throttle-pitch coupling under power. See the video below for an example of how the plane is supposed to behave when it's trimmed & balanced correctly, and the decalage is about right. Be sure to turn up the sound, as I narrate during the flight. It's a bit long, but the 19 minutes it takes to watch the entire video will most likely be worth it for those who are new to sailplanes.
Here's another method of fine-tuning the CG:
1) Take the plane up high, trim for level flight @ say 20-25% throttle. Better yet, trim for the flattest stable power-off glide (no porpoising).
2) Enter a power-off dive at a 45-degree angle, then release the sticks, and observe the plane's behavior:
A. Plane pulls sharply out of the dive as soon as you release the elevator: The plane is extremely nose-heavy. Move the CG aft, take the plane back up, re-trim as described above, then repeat the test.
B. Plane starts to dive more steeply (tucks toward the belly) when you release the elevator: The plane is tail-heavy. Move the CG forward, take the plane up, re-trim as described above, then repeat the test.
C. Plane continues on its path: The CG is at the neutral-handling point (often called the sweet-spot), which provides the greatest sensitivity to lift (the plane will indicate weaker lift than it will when it's nose-heavy). A neutral CG also results in the lowest stall-speed & the least amount of drag. But it also makes the plane neutrally-stable in pitch, so it will not self-correct in a dive. Many competition sailplane pilots prefer a neutral CG, but some pilots prefer a slightly-forward CG.
D. Plane starts to gently pull out of the dive when you release the stick: The CG is just slightly forward of neutral. The plane will still indicate light lift (albeit not quite as well as it would with a neutral CG), and it will be more stable in pitch than it is with a neutral CG. Some sailplane pilots prefer this CG position, as the slightly positive pitch-stability makes the plane less-apt to suddenly enter a dive or climb on its own when it gets perturbed by the air currents. The slight loss of efficiency is rarely noticeable to casual sailplane pilots who are not concerned with getting every last bit of performance from the airframe.
Remember that at this scale, moving the CG just 1-2mm is noticeable in flight. Avoid making large changes unless you're sure that the the CG is way off. Also, remember to re-trim for level flight each time you move the CG. This is absolutely critical. Failure to re-trim as described above will negate the entire process.
Also remember that CG is not a fixed number. Move it forward a bit for better wind-penetration on turbulent days, or for increased glide speed on windy days. Move it back to the sweet-spot in calm conditions and/or when lift is very light & you want to stay aloft on the farts of field-mice alone.
If the decalage is correct, you'll end up with the elevator level or close to level with respect to the stab after the above tests. If not - then the decalage is likely off. Which means you'd need to do the decalage test.
1) Zero your elevator trim on the tx, then mechanically neutralize the elevator with respect to the stab.
2) Take the plane up, but do not re-trim the elevator in flight. This is absolutely critical, as re-trimming the elevator after neutralizing it before the flight will screw up the test & render the results useless.
3) Perform a power-off vertical dive from high altitude, release the elevator stick, and observe the plane's behavior:
A. Model continues straight down: No change needed.
B. Model pulls to canopy: Increase stab incidence with respect to the wing.
C. Model pulls to belly: Reduce stab incidence with respect to the wing.
Adjusting the CG as described above is the next step after verifying that the decalage is correct.
UMXR flight-trimming and flying tips, power-on and power-off flight demos of how the plane should fly when trimmed and balanced correctly, power-off maneuverability demo. Be sure to turn up the sound, as I narrate during the video: