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Are there any charts or equations I can use to determine compensation for shorter scales when placing the bridge for classicals? I'm making bridges now for 2 guitars. One is a 640mm scale and the other is a 632.5mm scale. Trying to decide straight saddle slot or angled, but without knowing the total compensation I can't make a decision. Wendy

The more tension on the string, the less compensation it needs. There's less compensation on a 25.5 that on a 24.75.
Well, that said, a huge consideration here is the string gauge you will be using.

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Francis Richer, Montréal
Les Guitares F&M Guitars

Actually, that brings up another question. On the 640 scale I will probably be using normal tension D'Addario EJ45. Am I right that I will have to use high tension strings on the 632.5mm scale?

The tension you will use depends on more things than just the scale length. Lightly built short scale instruments may still sound best with lower tension strings. A large bodied, long scale instrument may sound better with high tension strings, (and often does).
If an instrument feels too "tight" with high tension strings, then try a lower tension set.

The 640 will be fine with normal or medium tension strings. The 632.5 I might consider high tension, particularly if you're going to use EJ46's. That loss of 7.5 mm will, somewhat, impact the tension of the strings at that length. They'll get to pitch before they reach their optimum tension if you use lower tensions at that length, I believe. However, you won't hurt anything trying mediums to see how they sound. May surprise you.

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Waddy

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Sound Clips of most of my guitars

It's not often that someone requests charts and equations, especially for string compensation! The good news is yes, equations exist; the bad news is that you need a lot of data (string material properties, action at every fret, scale length, etc. etc.) to feed the equations. And, of course, the answer differs with every different set-up and string type. Unfortunately there are no simple answers that are really effective. There's 25 pages on compensation techniques in "the book" (Google Trevor Gore guitars) or you can read about it here:
http://luthiersforum.com/forum/viewtopic.php?f=10101&t=33211&st=0&sk=t&sd=a.

If you find that the 12th fret intonation method is satisfactory for you, the quickest way to an answer will be to dummy up a plank with a tuner, a nut, a (part) fretboard and with a moveable bridge. Set the 12th fret action to what you like with the strings you prefer and move the saddle around until you get the note on the 12th fret matching the harmonic. Then copy that compensation length onto the new guitar for each string.

If standard 12th fret intonation drives you nuts because too many notes are out of tune on the lower frets, you have to resort to more exotic methods, but I'm not going to write 25 pages here!

Trevor Gore.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

As far as angled saddles go, I now use them because in theory it makes sense however to be honest my older straight saddle guitars are intonated just as good (or bad) as the ones with angles.

If you follow standard practice you'll get good results. I might suggest using a saddle wide enough to allow you to fine tune once the guitar is strung up if you really want to get into precise intonation.

As for strings, I find it good practice to start with a constant string type on each guitar and move from there. I usually use EJ45s which are normal tension and fairly neutral. They are also widely used and have become a de facto standard. Any changes that you apply are only useful when you can say "compared to...".

I have really been appreciating the Aquila Nylgut strings for lightly built guitars. They add a lovely sparkle to my Torres style builds that regular nylon strings just don't.

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Expectation is the source of all misery; comparison the thief of joy.
http://redrivercanoe.ca/

Wow, I was surprised to see this post resurface, I had given up on getting any more input on the topic. Lots of information, thankyou. I went ahead and cut the saddle slot straight. What I am looking for is a general answer, on how far back to set the bridge for the 632.5 mm scale, when I glue it on. I can always fine tune the saddle when I do the final setup. I really don't want to go to the trouble of making a dummy plank and movable bridge to get an answer for a one off guitar.

Waddy, I will try the medium tension on the shorter one first, since it is for a friend who prefers lighter tension. We will see what they sound like, thanks.