To calibrate your hygrometer you will need table salt, a see-through plastic bag such as a ziplock bag, and a small container such as a bottle cap.
Place a teaspoon of table salt into the bottle cap and add a few drops of water to dampen it. DO NOT dissolve the salt, the goal is to just get it slightly wet. Then place the bottle cap into the ziplock bag along with the hygrometer. Seal the bag with some air trapped inside and let is sit (approximately 6 hours). After it has stabilized, check the bag without opening it. The hygrometer should read 75%.
If your hygrometer does not read 75%, you can do one of two things. Your first option is to be aware of the deviation of your hygrometer. So, if your hygrometer reads 85%, then you are 10% off and have to subtract 10% from whatever it reads. Your second option is to adjust the hygrometer. Some hygrometeres have a dial in the back or the side that allows your to adjust the unit to 75%.