I think I’ll try tuning them up to E and see if it makes a difference. 

@dusty-turtle interesting!  I ordered a basic FolrkLife set with 11s for my melody strings and not being used to tuning a dulcimer, broke them. So now I have GHS Boomer electric guitar strings in 13 (I think B string on guitar in that set) on the melody strings as I had a bunch of sets laying around I bought in a blowout sale once. 

decided I liked that better anyway, especially trying to tune for Joni’s All I Want. Will look at the middle and bass strings too and see if I want to up the gauge.

Lighter strings will be looser.  Heavier strings will be tighter.

If you're playing by yourself, though, and don't need to be in a specific key, I would suggest tuning higher for tighter strings and lower for looser strings.  That's a lot easier than changing strings.

I have a follow up question. I know I’ve heard comments about floppy strings with low/loose tuning. I just can’t remember what the cure is, heavier or lighter gauge strings? Thanks

Great answers to a great question!

The calculator will help you avoid putting on strings that would be either too slack or so tight that they might break. It's so useful when you want to try out a new tuning or key for a particular sized dulcimer. (btw, it works well for other stringed instruments as well, not just dulcimers!)

Traildad --  exactly right -- string gauges calculators give you as usable string gauges 

String gauges are  like many, many things "dulcimer", there is no such things as "the one and only" answer.    You've seen several times now that there are no hard and fast rules, no "best", no absolutes. 

Each of us approaches the dulcimer differently, with clusters of people in several different (sometimes wildly different) groups.

The proper answer to most questions is usually "Whatever works for you."

I’m glad I asked. I had gotten the impression or made the assumption that it was like using a calculator to figure what 2+2 is. It sounds like the calculator will give you a useable string but not the one and only. More of a starting point than THE answer. Thanks for the info. 

Perhaps this metaphor will work.  Imagine your first time ordering a steak at a restaurant. The waiter asks how you'd like it cooked.  Obviously, the extremes won't work, for fully uncooked could get you sick and completely burned will be a bunch of charred carbon with no meat.  But in between there is an acceptable range from rare to medium rare to medium to medium well to well done, with variations in between.  Maybe you ask what most people choose and you are told "medium" is the most common choice, but just to be safe, you should ask for it a little more rare, since you can always put the meat back on the grill.  That advice is like the string gauge calculator, which errs on the light side.  But then you have to develop and identify your own preferences in relation to that.  So you experiment.  You try a steak medium rare or a string a bit lighter than the gauge indicates, and you see how it fits.  Then you try a steak medium well or a string a bit heavier than the gauge indicates, and you see how it fits.  You have to experiment and decide how you like your steak, or how you like your dulcimer to sound and to play.

Like @susie, I find the string gauge calculator significantly lighter than my preferences.  Tuned DAd on my main dulcimer, I use .026 wound on the bass, .016 on the middle, and .014 on the melody.  And that's for a dulcimer with a pretty short (25") VSL.  I like the heavier gauges for the increased volume, clearer tone, and ease of controlling techniques such as bending strings.

Sometimes, such as when I fingerpick, I like a little extra give in the strings, but instead of changing strings, I just tune down a half step or whole step to C# or C. And when I play fast flatpicking tunes, I want the strings really taught so they are more responsive to the pick, so I tune up to Eb or E.

These are my preferences, and as @ken-hulme explains, you will have to develop your own.  And I make some adjustments depending on the dulcimer I am playing as well.  Some instruments seem to do better with heavier or lighter gauges.  Over time you will figure this stuff out for your instruments and your playing style.

I would suggest buying a whole bunch of strings and playing around.  Once you know how you want to set up your dulcimers, then you can buy strings in bulk and you never have to think about it again.

I'll buy sets of strings in common gauges and packs of singles in light gauges and use whatever I have on hand for a re-string.  Using all light gauge, for example, I might have a .014 for melody then .012 for the drones.  I always have .022 or .024 on hand for when I want to switch-out a high drone for a low one on the "bass" side of the instrument.        

I've never relied on the string gauge calculators. When I've tried them, they have always seemed to be too light for me. I've always started with what the maker of my instrument recommends, in the tuning that I use, then go from there. After playing for awhile, I determine if a change is necessary, based on how the strings sound in relation to each other and how they feel on the instrument (where VSL matters).  I just recently adjusted just the 1st and 2nd string gauges a little heavier on one of my dulcimers, which made a improved change in the sound. The other thing that makes a difference for me is  the makeup of the string. What might sound fine on one instrument, sounds awful on another. Strings are cheap and experimenting is all part of the process.

How do I choose strings for my instruments?  VSL, the open tuning I want to use, and years of experimenting and experience.  That's what the builder of your second dulcimer used -- his experience with various gauges of strings to produce a sound which he likes.  If you are not experienced in such things, the recommendation of the builder is always a good place to start.

You asked "Now I’m wondering if there is more to choosing string gauges than running the string gauge calculator."   Yes. Of course.  But that's the part which comes from experimenting and experience and deciding what sound YOU like to hear.  Not me. Not Chuck, or Mary, or Jim.  What YOU like to hear.  

All  my existing dulcimers are strung the way I like them, I don't experiment much any more -- I know what I like.  When I finish a new dulcimer, I start with a string calculator, of course.  My personal preference is the Strothers calculator, which is known to be noticeably light in its recommendations.  So I step up at least two gauges -- 10s become 12s, 12s become 14s etc.  My preference for bass strings is plain steel not wound for a more traditional sound.  

I’ve learned that it matters what string gauge is used and it depends on the VSL/scale and what key you are tuning to. Then I found out about online string gauge calculators. I’m wondering if most people use the string gauge calculator or something else. Maybe whatever size came on their dulcimer when new or a gauge they decided sounds best. My first dulcimer came with extra strings .024, .014 and .012 for tuning DAdd. The string gauge calculator calls for .019, .012 and .009 for DAdd on the 28.5 VSL. I didn’t know what strings were on my second dulcimer when I got it. After experimenting with new strings and tuning I wanted to return to the original DAA tuning. I checked the calculator and ordered .018W, .012, .012 for its 30” VSL. After playing it I thought something was different. I asked what size strings were on it new and it was .026, .014 and .014 because he thought the heavier strings improved the tone and volume. I haven’t switched back yet but I agree it sounded better with the heavier strings. Both dulcimers have bodies on the larger size. Maybe that is why both came new with heavier strings. Now I’m wondering if there is more to choosing string gauges than running the string gauge calculator. How do you choose your string gauge?