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I got the french polish all laid down and am ready for the bridge on a little terz guitar.
It'll be tuned up G to G and strung with 12-53
Scale length is 548mm.

I've used three different calculators.
Stewmac, Liutia Motollo and one other.
Getting differing results on all three.
One says 3.2mm-6.3 another says 1.9-4 and the other came up with 12mm so I know it's wrong.

What's the hive mind think about compensation on this short scale little gem?

note- tension is EXACTLY the same as standard tuning on a 25.4" scale length (sorry for mixing metric and empirical. We luthiers are weird that way)
As it is a 25.4 fingerboard cut off at 3rd fret. So it's like a capo at 3rd fret. Same strings. Same tension. Same tension.

TIA for advice.

dave

My first thought is if it's like a capo at 3rd fret on a 25.4" scale, why wouldn't it be the same compensation as normal for the 25.4" scale as long as the nut slots are well cut.?
But I've given up cutting the bridge slot before gluing it on the soundboard, I slot on the guitar using a maple "false saddle" intonating on the E strings, (leaving a little to cut back the saddle at the low E)

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Great observation!

When doing a new design, I make a sort of fake tailpiece with the outer strings in the correct string path, set the string action height where I want it, and then float the unglued bridge back and forth with the standard saddle I use until the intonation is spot on in playing position, then reverse engineer a bridge locating jig from that…

How many of the strings will be wound, versus plain?
As you are probably aware the plain strings will need longer scale.

The StewMac calculation is a very rough approximation for typical guitar configurations, which a terz is not. The Motolo calculator, if its the one I remember, should be quite good. Can't comment on the third calculation.

Some time ago, I prepared an Excel calculator for compensation calculations in the context of a multi-scale instrument, including baritone. The references for the formulae are given in the sheet. From my testing and that of others, this calculator gives VERY good results, but does require more physical data for the strings than is typically found online, even on the D'Addario site. If you use the tension data for D'Addario 12-53 strings, you can sacrifice a set to derive the other weight/m and core diameter data needed.

The calculator is an Excel spreadsheet. A zip file is available from a link in this post: https://www.mullinguitars.com/2015/10/compensation-calculator-for-steel-string-guitars.html