Fret Calculator.All fret distances are measured from the nut.
Compensation (offset) does not change the actual string length as measured from the nut to the saddle.
For the purpose of correctly calculating the taper, the unit of measurement for the nut MUST be inches.
Other dimensions may be in metric units (consistently) if desired.
DimensionsLet us begin with the gathering of some information.
This information should be gathered from the customer.
Defaults are in square brackets: 
Asterisk (*) indicates item should NOT be changed without expectation of corresponding change in tone or resonance.
A sample custom instrument (I call the Blond model) is also included here and is indicated by the use of the color blue in materials and dimensions. This sample custom instrument is a suggested alternative designed in lighter colors and with a slightly smaller size.
If changes have been made, the instrument plans should be re-drawn, as new calculations will be needed throughout. Changes in materials may have profound effects on the method of carving the woods and possibly the gluing as well. It is therefore recommended that the plans not be altered by the builder until after experiencing the construction of an instrument built to the default specifications.
The following notes will guide you in customizing the dimensions. Starting at the bridge and assuming the majority body will be left with the same dimensions, we will need to decide the where the 12th fret and therefore the top shoulder of the body is at. If the new neck is longer (or conversely, shorter) by some planned amount, then the body will be longer (or shorter if the neck is shorter) by half this amount. In the case of the Blond version (above dimensions) The neck will be shorter than the default dimension by 0.947 in. so the body will be shorter by half (0.474 in.). The length from the tang end of the neck to the nut will be shorter by the entire amount (0.947 in.) And the position of every fret will need to be changed, thus the end of the fret board will also be slightly different and the top will need the length of its notch changed to fit snugly and not have a gap when it is later set in place. The strings spread out going from the nut to the bridge and the neck follows the same line (parallel, about half of the string to string distance away from that outer string.) And this line is changed when the length of the neck or the width of the nut is changed. It is important to determine this line accurately and from it, determine the width of the neck at the 12th fret for this is the proper width for the tang and its slot in the body. Trigonometry is a must for those who would calculate this correctly. When machining the fretboard, the exact angle must be known in order to taper the sides of the fretboard correctly so that the outer strings will each have just the right amount of fret under them. Using a different pickup would possibly change the string spacing at the bridge and also require the recalculation of the fretboard taper angle and the width of the tang. The reason the tang width equals the fretboard width at the end of the body is to make a clean (not stepped) slot opening on the body where the neck sets. I strongly suggest consulting several sources (books, articles, other luthiers) regarding placement of frets with respect to string length, action, and string diameter (and the string mass too!) before experimenting. Don't guess at these dimensions, but take full advantage of the accumulated knowledge derived from centuries of builders experiences. Modern luthiers often are very scientific in their approach and their acoustic measurement tools exceed the sensitivity of human ears. For an experiment to be of any use, there must be a control to measure the departures from, and the changes must be simple and one dimensional or the data may be of little use. (If you change; string length, action, pickup, neck wood, and fret wire all at the same time, it will be impossible to say which is responsible for that cool change in tone from your previous instrument. On the other hand when enough data has been analyzed an equation may be written which includes all the variables, and you may then solve this equation perhaps using a spread sheet to compare a number of complex sets of parameters.)
Cocabolo is a dangerous wood to work with (it is an irritant and I have seen people get physically ill from breathing its dust. Wear a dust mask.) Yet is responsible for the incredible sustain of this instrument. The neck is unusual in being made of this wood as opposed to the normal light woods used. While it may be possible to substitute another material, it is recommended this material be selected based upon it's specifications compared directly with that of Cocabolo.
Spruce was chosen more for it's traditional look than for any other reason. Cedar is also accepted as a material for tops, but most other woods are not, and a strong wood, light of weight and color, is recommended.
The back and sides are most likely to be changed, but it should be noted that they should be the same wood or there will be a noticeable two layers visible from a side view. A small groove is cut where they are joined and this is to be filled with binding of like color.
Old jade is nepherite, and seemed perfect for making a nut of. Substitute ox bone if you wish, or visit a rock hound and ask about Wyoming jade (the DARK green kind is sometimes almost black, and is the kind you want, not the light colored jade which is a different mineral even though also called jade.)
I believe John Gilbert makes the best tuner. (He told me that he once was chief tooling engineer for Hewlett Packard.) The headstock is designed with them in mind. If you choose other tuners, be cautious and check your dimensions as you may need to redesign the headstock to accommodate your tuners. Again, it is recommended these tuners be used for a luthier's first building of this instrument so that they will have a functional working model for reference when later building custom versions.
Our basic process will consist of cutting parts to size, fitting them together, finishing the instrument, and then setting the instrument up.
Our basic wood parts will consist of a body into which the neck will be glued, followed by the top and then the bridge. Nine (9) knobs will complete our list of wood parts and should be made concurrently with the other parts so that the entire project may be finished (coated) at one time.
It may be necessary to laminate some of your wood pieces to obtain large enough (composite) pieces before cutting to size. Pre-machine (cut) these smaller pieces before gluing to form the larger pieces. Pay attention to the matching of grain patterns and the squareness of your cuts, then use a glue recommended for that type of wood. All raw wood is assumed to have been purchased just a bit over sized and then machined (cut, sanded,) to the sizes indicated. It is wise to begin with stock which has been squared and sized as you may later wish to rely on measurements taken directly from these surfaces, so this can be extremely important.