A friend who has played a "traditional" lap dulicimer (no 6-1/2 fret) for many years, has the ever-popular quote from an early devotee pasted in the lid of his case, and frequently points it out to others. My recollection may not be entirely accurate, but it's something like:

"You don't tune a dulcimer, you just plays them."

The "traditional" response to how to tune a dulicimer is:

"Find a pleasant tone."

A particular problem with the lap dulcimer is my (too frequent?) reminder to my beloved one:

"It does help a bit if it's the same pleasant tone someone else is using."

John