First this is not a mandolin question as mandolins usually have floating bridges, so I don't have issue like the one below:
I bought a brand new guitar (classical, nylon string) last December. After playing for 3 months, I realize that the intonation is not that good, I can heard notes and chords being a bit off-tuned. At fret#12, the chromatic tuner shows that all the 6 strings are sharp, and I can hear the notes being sharp too.
So I measure A = nut to fret#12 (the fret, not the wood)
and B = fret#12 to the tip of the saddle where the strings make contact
I found B = A + 0.5mm approx.
From my tenor ukulele building experience, I had to give B = A + 2.5mm so I have room for saddle compensation and its intonation at fret#12 is near perfect, the tuner shows exactly one octave higher.
Since the guitar has fixed bridge, what would be the best recommendation to remedy this intonation issue ? (without having to unglue the bridge and reglue it at the proper position, I would create a mess on the top sound board if I had to do it this way).
BTW, this guitar has an under the saddle transducer (classical , electric).