The salt test NaCl isn't that useful for guitars. However, K2CO3 Potassium Carbonate is -- a saturated salt solution maintains a RH of 43%.
The next time I calibrate my hygrometers, I'll do a pictorial as I see this question asked a fair amount. However, rather than going to the trouble of doing all this -- just buy anew Caliber IV hygrometer every 2 to 3 years and you're set. They are calibrated very well and the calibration will be suitable for a couple of years.
If you want to use Potassium Carbonate, you can also buy that off of Amazon. And that way you can get about 5 or so years out of your digital hygrometer by calibrating every year or so.
If doing a salt test, you want to use a sealed container - Tupperware works great. Place a tbsp of the salt and then add water until you get a slurry. You want to see water but with undissolved salt -- that way you know it is "saturated". I let it sit for a day and check 24 hours later.
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