Here's a picture of Jim Good (Mastertone) 5 string with doubled bass.

Thanks everyone for your thoughts and the great information.  You guys always come through!

I have only built a few five string dulcimers, but have never doubled the bass string.  Doubling the bass string gives way too much bass (I tried it once on a six string dulcimer).  I have always doubled the melody and middle string.  So my spacing suggestion is a little different.  Outside melody 1/4 from edge, next melody 1/8 further in.  Bass 1/4 from other edge.  Measure halfway between inside melody and bass.  But middle string 1/16 to either side of the center position.  If you prefer a little more distance between strings, change to only 1/8 from edge of fretboard to first melody and bass.  As always, adjust measurements to taste.

With the tuning head to your left, closest to you will be two melody strings, the first one is 1/8"in from the edge of the fretboard, the inner string 1/8" closer to center.  On the far side of the fretboard, you have the same situation -- the outer bass string is 1/8" from the far edge of the fretboard and the inner bass string 1/8" closer to center.  The middle drone is half way between the inner melody string and the inner bass string.   Generally speaking, as Skip mentions, the bass couplet has a regular bass string appropriate for the VSL, and the inner bass string is the same as a melody, and intended to be tuned an octave above the bass string.  From bass to melody,  the usual tuning setup is  DdAdd.

You can buy commercially made nuts/bridges from McSpadden, Folkcraft and elsewhere which already have the notches cut for 3, 4, 5 and 6 string layouts,

It's probably going to be 1/8" from the sides to the outside strings, another 1/8" between doubled melody and bass strings and  single middle string. The bass set is probably a bass and its octave with doubled melody strings. My 5 string Blue Lion is 5 equidistant strings, 1/8"  sides to outside strings. 

Look really close at where the strings connect to the pins also

updated by @skip: 06/25/22 06:51:28PM

I'm the new guy here.  I acquired a five string dulcimer as a restoration project.  It's missing the nut and bridge so I have no idea what the string spacing was originally.  I'm guessing it probably had two double strings and a single bass but can anyone recommend the proper spacing for the five strings?  The fret board is 1 1/2 inches (39 mm) wide.  Thanks for any help you can provide.

updated by @steven-stroot: 06/26/22 02:08:10PM