I got a Suzuki MRT-01 with my Melodion to help with tuning and maintenance. The availability of the MRT-01 and the information on Suzuki’s Melodion website is why I went with Suzuki. I followed the instructions that came with the set.
Here’s a link to the instructions (page 1 is Japanese and page 2 is English): http://www.suzuki-music.co.jp/search/files/002018_3.pdf
See part B for information on reed clearance. Also, notice the “⚠️ NOTE” at the bottom of part C where it says “Recheck the reed clearance after tuning as it may change”. It’s also important to note that the Japanese and English do not exactly match up. There is no mention of ‘custom reed clearance’ in the Japanese instructions.
Incidentally, with the spatula provided, I found that I could be relatively aggressive with my reed clearance tuning. I changed the clearance on some of the same reeds over and over and they showed no signs of breaking.
It’s also worth noting that one should use their main mouthpiece while tuning as air pressure can slightly change depending on the mouthpiece used.
Re: What I think about how to produce a balanced sound/Doesn’t it depend mostly on playing
IMHO, I think of a melodica as a solo instrument (as in its main use should be playing single lines). This has to do with the fact that the mouthpiece is not equidistant to every reed therefore creating an uneven distribution of air pressure causing a difference in tuning between notes while playing chords.
Imagine, for example, a stringed instrument whose open strings are slightly out of tune- that would be similar to uneven reed clearance. IOW, the player would have to compensate either by playing a string sharp (either by bending or by position) or flat. To me it makes more logical sense to fix the open tuning (or the reed clearance in the case of the melodica) to be relatively in tune with each string.
Unfortunately, this concept suggests some negative (and inevitable) implications about the melodica. A change in dynamics ultimately affects tuning. The recorder (which is another instrument that is extensively used in Japanese elementary school music education) also has this problem. Thankfully the melodica isn’t as sensitive as the recorder is in this regard.
If you have the money (and more realistically, the time) then by all means get a spare melodica and some tools and test out a more aggressive reed clearance/tuning approach. Ideally one would have some kind of air pressure machine to test each reed with a consistent amount of air pressure. But those machines can be noisy and expensive.
Another approach is to use a decibel reader. For example, you can use 80db (or quieter depending on how you like to play) as your reference to help determine if reeds are sharp or flat. Realistically this is a starting point. Once you’ve set the reed clearance so every reed is in tune at the same volume, you’ll want to fine tune the reed clearance so the lower notes are louder (according to the decibel reader) than the higher notes. This will create a consistent volume according to the human ear.
I hope that helps!