Greetings,
I've been doing pro intonation setup (guitar/bass - electric & acoustic) for over 3 decades.
Most luthiers and big-name factory setup specialists I know will intonate at the 12th fret and stop there. This is standard practice in the industry and works for most pro-level guitar/bass players.
But there are some serious players (like YOU) who demand more accuracy in tuning the notes above 12th fret. This is where I come in. I have always been told that the 14th fret note on the G string cannot be intonated perfectly and will always be slightly sharp no matter what.
So one day, I grabbed a cheap classical guitar that had really bad intonation and used a nail file to sand the bone saddle one string at a time. I went through five sets of strings in the process and the result is amazing! I was able to get that 14th fret A in tune without throwing the intonation on other strings off. The guitar sounds beautifully in tune now.
After having gone through the above mentioned process, I figured out all of the focus points to achieve perfect intonation in a timely manner with just one set of strings.
It's an extremely tedious work to do this level of intonation accurately but I love doing it! There's only one guy that I found on YouTube
that talked about this kind of intonation setup, he was the owner of Suhr Guitars - he called it the "$100 Setup."
If you have a guitar (electric or acoustic) that plays the A minor chord in tune perfectly at the 1st position but is out of tune at 13/14th frets, then I can help you!
You could spend $75 now to get your guitar accurately set up and intonated or spend the rest of your life trying to tune it.
Please reply with a detail of your guitar's intonation issue and we'll take it from there.
Thank you.
Best,
Shiro