Eeek! Yes, calibrate it.
Put a good-sized (1/4 cup?) pile of salt in a bowl - NOT the iodized kind. Put enough water into the salt to make "wet sand" consistency, not "soup" consistency. Put the bowl of wet-sand salt and your hydrometer into a ziploc bag and seal up the bag. Leave it sealed for at least six hours - 8-12 is better - and your hydrometer should be reading 75% humidity.
If it's reading, as mine did, 68%, then from now on add 7% to whatever the reading is, to adjust for the difference. I made a note on the screen of mine with a fine-line sharpie marker so I would remember my adjustment amount. ("Add 7%" printed on there.)