To check if your hygrometer is reading correctly, use the salt method to test it's accuracy. You just need to...

Place some salt in a bottle cap or small cup and dampen it with a few drops of water with out dissolving it.

Carefully place the damp salt into a see-through container and close it tightly. You can use a zip lock bag provided it seals you leave some air inside the bag.

Let it sit for at least 6 hours and note the reading on the hygrometer with out opening the container. It should read exactly seventy five percent. The difference is how much your hygrometer is off.

if needed adjust the seventy five percent, using the screw on the back of the hygrometer.

A digital hygrometer is the best for your humidor as it will provide you with the most accurate humidity level.