Fill a bottle cap with a teaspoon of salt and moisten with a few drops of water.  Make a paste, not a saline solution. Place the bottle cap along with the hygrometer in a Ziplock bag and seal it up.  Let it stand for about 6 hours then check the humidity level.  It should read 75%.  If not, you can adjust most analog hygrometers with a screw on the back of the unit.  However, some hygrometers don’t have an adjustment screw so you’ll just have to note the difference and apply it when you check your humidity level in the future.