Consider this fret ruler. allows for fanned frets and scala files for weirder setups.

https://inkscape.org/~Neon22/%E2%98%85luthier-fret-ruler

Wow, that is one beautiful piece - and how on Earth did you get those inlays done?! Very excited so see how it will sound.

What tuning do you have in mind?

Thanks!

The inlays were done on the scroll saw... it sounds complex but is pretty simple.  The basic technique is as follows:

-Stack the inlay (maple) on top of the container piece (wenge).  In my case they were both about 1/4" thick.  I used painters tape and a glue stick to hold them securely together (but not permanently).

-Glue / tape the template (what you are cutting) on top of the inlay piece.

-Set the scroll saw table to a slight angle (based on the blade that I was using and the thickness of the wood, I found that about 2 degrees was correct.

-Cut around the template, such that the angle of the bed causes an inward taper (i.e. the bottom of the cut is smaller than the top)

-Once the inlay is free, remove the maple from the bottom piece.  The maple will slip into the wenge hole, and since it is tapered, it will fit tightly.  (This is why you need to adjust the taper, so that the inlay does not drop too low.  You want it to be slightly proud in relation to the wenge, so that it can be sanded flush).  Glue it in and sand it flat.  

-Once dry you can fill any holes (the drill hole for instance) or gaps with white glue + sawdust, and re-sand.

As for tuning, I am tuning it like a baritone ukulele / top 4 strings of the guitar, in DGBE.  However I am considering playing a bit with dropping the D and / or G strings to be one octave lower (i.e. D2 instead of D3) to give the thing a bit more low end.  I just need to pick up a thicker string today, as the light gauge low E string I have (0.042") is not thick enough to give a good sounding D2 on a 24" scale length instrument.  I am thinking that 0.050 - 0.058 should work fine... just a matter of picking up another set of strings.

Cheers

I'm biased, but have a look at D'Addario, they have quite a few hybrid gauge sets like the Bluegrass ones for steel string (EJ14 or EJ19, depending on the material). Love them on my Dreadnought, and should give you both options with the G string. Well now I'm even more excited to hear it :)

And thanks for the details on the inlays, it seemed so obvious once I started reading..

Yeah, those sets do look nice.  I just picked up a set of store-brand light weight strings initially as they were dirt cheap, but I'll probably end up springing for some nicer D'Addarios or similar next time.

I have been experimenting yesterday evening with using the low E string (0.042") as a G2.  The tension is good, but it doesn't feel / sound quite right.  Not sure if that is due to the non-linear progression or if the small body is just not well suited to deeper notes.  I guess I should just try picking up a single thicker string (my local music store has 0.054 and 0.056 ones) and see how it works for a low D... worst case I don't use it, and am out $3.

Could it be the intonation? Well, this is quite a non-standard instrument, so it could be a lot of things. Hope you don't have to start filing around on the saddle and different string gauge will do the trick.

The intonation appears to be fine (I have a floating bridge, and adjusted it when stringing to get the 12th fret as close as possible to being dead on the proper note).  It is slightly angled, which is how they are supposed to be, so the theory lines up with practice as well.

The setup still does need a bit of work (the high E string is just a tiny bit too low at the nut, while the D string could probably go a bit lower), but it is decently close.

Yeah, with something this customized I really have to step back from specifics and look at general theory... it is hard to even say what to call this thing, let alone how to troubleshoot non-standard tunings!  (On that note, I think that I have found the nearest commercial cousin to this thing - plectrum guitar, which is essentially a slightly larger version of a tenor guitar - 4 strings, but with a longer scale length.  My scale length of 24" seems to fit right between a tenor guitar (21 - 23") and a plectrum guitar (26 - 27")).

Anyway, I am still going to try with a dropped D string (so D2 G3 B3 E4) and see if that works any better... if not I will just go with standard Chicago tuning.  It does sound pretty nice like that IMHO.

On the positive side, since I made the nut and saddle out of an old horse bone, I have plenty of raw material left over if I need to re-do them. :-)

There's something poetic in here about the dead horse that ended up as saddle..

But hey, with a guitar this unique, you can call it whatever you want :)