Before you open anything
Tune the instrument closed, playing normally, exactly as it will be played. A reed measured with the block out and the case open is a reed measured in a different instrument.
Measure the reference, do not assume it
Fix A4 before the pass starts and leave it alone. But fix it to this instrument's A, not to a number you like: play its A4 and read what it actually is. New boxes are often 442, plenty are 440, and old instruments turn up at 435 or 432. You are tuning the accordion to itself.
The app freezes the reference for the length of a pass, because a pass measured against a moving reference tells you nothing you can file by.
Set the tolerance
Tolerance is the line between "leave it" and "touch it". It belongs to the instrument, not to you: a concert box is held tighter than a folk one. Set it once on the instrument and let the app judge against it rather than judging each number by eye.
Recording the pass
Work the keyboard in order, one note at a time, holding each until the reading settles and locks. Keep the bellows pressure steady and roughly what the instrument gets played at.
The recorder stamps every reading with the bellows direction, so set the direction before you start and change it when you turn around. Every note has a press reed and a draw reed; they are different tongues and they drift apart on their own, so both get recorded and both get tuned. The direction control sits on the toolbar once a session is open.
If you would rather be told what to play than remember where you are, the calibration sweep walks the range for you note by note.
Reading the errors
The table gives you, per reed: where it is (Curr), where it should be (Goal), and the distance between them (Error, printed as Err on the report). That distance is what you file by. Amber means the app has judged it out against your tolerance.
You can have the app say this either way round, in Settings: read Curr and drive it to the goal, or read Error and drive it to zero. Same measurement, two conventions. Pick the one you have in your hands.
What not to fix
A wide beat in a musette is not an error. If the beating curve says the pair should beat at two hertz and it beats at two hertz, it is right, however far each reed sits from the bare scale.
The same logic runs the other way. Two reeds both three cents sharp still beat at exactly the right rate, and nobody will hear it. The beat is what the ear notices; hold that tighter than either reed's own pitch.
Filing: do not file to zero
This is the part that costs reeds.
The moment you open the accordion and lift the block, every reed's pitch moves - by around three cents, which is several times your tolerance. It moves back when you reassemble. So the number in front of you on the bench is not the number you are tuning to.
Work it out from the report:
target on the table = what it reads on the table − the Err from the report
The table reading is your working number. The report supplies the offset. File until the reed reads the target on the bench, put it back, and it will be right in the box. Filing to zero on the table guarantees it is wrong once assembled.
Which reed first
Within one note, bring the reference reed - the one your curve puts at pitch
- to its own target first. Then bring the others in against it until the beats are right. The reference reed is tuned absolutely; the rest are tuned relative to it. Which reed that is, is yours to choose; nothing in the app assumes it.
Which way to file
Metal off the tip lightens the free end and the pitch rises. Metal off the base makes the tongue more flexible and the pitch falls. Take it evenly across the width, and do not score a sharp notch: an abrupt change of section is where reeds break. Beyond that, the filing is your craft and not the app's business - SmeggTuner's job ends at handing you a number and its sign.
The second pass
Close it, record again, and lay the two passes over each other. The comparison is the point: what improved, what got worse, what fell out of tolerance, and what you can stop touching. It is the only honest way to tell whether an afternoon of filing helped.