A guitar can be perfectly in tune on its open strings and still sound out of tune higher up the neck. That is intonation: whether a fretted note lands exactly where it should. The usual test compares the harmonic at the 12th fret with the note fretted at the 12th fret. Both should be the same note, exactly one octave above the open string.
This tool runs that test for you. For each string, play the harmonic and let the tool lock onto it, then fret the 12th fret and play again. It shows the difference in cents. If the fretted note is sharp, the string is too short: move that string’s saddle back, away from the neck. If it is flat, the string is too long: move the saddle forward. Using your scale length, it estimates how far: on a 25.5 inch (648 mm) guitar, every cent is about 0.37 mm.
Readings need to hold steady before they are taken, so a wobbling or dying note is ignored. A summary table builds up as you go, and you can save it as a PDF to keep with your setup notes.
Always check with the guitar tuned and in playing position, ideally with fresh strings, since old strings rarely intonate well. Loosen a string a little before moving its saddle, retune, and check again.