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.
Dusty T., Northern California
As a musician, you have to keep one foot back in the past and one foot forward into the future.
-- Dizzy Gillespie