Beginning dulcimer makers are often perplexed about knowing where to place the frets.
The first step towards getting a dulcimer to play in tune is to have the frets placed properly.
There are many discussions about temperament, scale length, etc.
Most current fretted instruments in the Western world are fretted to play in equal temperament. A dulcimer that is fretted to play using the equal tempered scale will play in tune with most other (Western) instruments.
When I began making dulcimers I was taught to divide the string length by 17.81715385 and the resulting number would be the distance from the nut to the first fret. I would then subtract the distance from the nut to the first fret and divide the remaining string length by 17.81715385.
Lather, rinse, repeat.
I would calculate all the frets for a chromatic fingerboard but only use the ones I needed for a dulcimer. This way I had the measurements for any extra frets I might be installing while making the dulcimer or adding to it in the future.
After doing this for a while someone mentioned that no one would hear the difference if I simply used 17.817. They were right. It saved lots of keystrokes on the calculator.
Now you can easily generate fret positions automagicaly!
Here is a list of some on-line fret placement calculators I have found either useful or interesting.