Correcting intonation

[mawmow]

The intonation is spot on four strings, but a little flat on a string and a little sharp on another : I could craft a new bony bridge to try to correct that, but I do not know which way, shorter or longer scale, to go to correct (at least partially) flat and sharp intonation. So ?
Many thanks guys !

[Mirosh]

Making the nut-to-contact point on the saddle longer will flatten the notes on that string. Shorter will sharpen them.

[mawmow]

Originally Posted by Mirosh

Making the nut-to-contact point on the saddle longer will flatten the notes on that string. Shorter will sharpen them.

Thanks a lot !

My question retrospectively seems to me so basic :
When I get a fret higher, I shorten the string and "sharpen" to the next note !
Anyway, I would have needed someone to tell I had it or not.

[John Arnold]

Frequency is inversely proportional to string length.

[Sasquatchian]

I found it's more helpful to tune the string to an open pitch, then fret at the twelfth fret. Then only consider that second half of the string and ignore the first twelve frets. If you're sharp, you need to make the rest of the string length longer, and just the opposite if it's flat up the neck. And on that note I've been making saddles from the general bone blanks from Stew-Mac which are too thick for either Martin or Gibson saddle slots, so I mark them and file both sides down leaving an overhang above the slot, giving me just a millimeter more room for adjustability. Of course that takes a couple of hours to get it perfect and have a nice snug fit but it beats waiting a couple of months for the repair dude to do it for me, and the one I made last weekend is even better than the one he made last fall.

[rick-slo]

Originally Posted by mawmow

The intonation is spot on four strings, but a little flat on a string and a little sharp on another : I could craft a new bony bridge to try to correct that, but I do not know which way, shorter or longer scale, to go to correct (at least partially) flat and sharp intonation. So ?
Many thanks guys !

On what basis did you determine flat or sharp? For example 12th fret harmonic versus fretted, or the mainly when fretting near the nut,
or on certain chords and not other chords? Not sure a new bridge is the solution to the issue.

[mawmow]

Originally Posted by rick-slo

On what basis did you determine flat or sharp? For example 12th fret harmonic versus fretted, or the mainly when fretting near the nut,
or on certain chords and not other chords? Not sure a new bridge is the solution to the issue.

I gently fret just behind the twelfth fret.

[rick-slo]

Originally Posted by mawmow

I gently fret just behind the twelfth fret.

Good. Which string is a bit flat and which a bit sharp and tested in standard tuning or some lowered tuning? Newish strings? Perhaps a compensated
saddle rather than moving the saddle slot. Good luck on solving the issue.

[mawmow]

Originally Posted by rick-slo

Good. Which string is a bit flat and which a bit sharp and tested in standard tuning or some lowered tuning? Newish strings? Perhaps a compensated
saddle rather than moving the saddle slot. Good luck on solving the issue.

Another member also warned me about string age effect.
I do not want to change the saddle slot, but craft a new compensated saddle.

I am not ready to do it right now.
I also plan to make an inventory of string intonations on all my acoustics :
It could become a cold season project.

Thank you, guys.