27" scale, tuned DAd.
Just ordered the nut and saddle and don't know how much I should sand them down as I'm not really a dulcimer player.
Thanks in advance
The optimal string height for a mountain dulcimer won't be radically different than for any other fretted instrument. It is at least partly a matter of personal preference. In general:
This article deals with building from a kit, but does address your specific question, down in Step 27.
Several players recommend a height about the thickness of a dime at the first fret, and about the thickness of a nickel at the seventh fret. The general strategy recommended is to bring the bridge and saddle a little bit at a time until you find a height that works for you.