1% change in the reading per click of the dial, no?

BTW, I noticed when I check the calibratation of my Hydras that the other Hygrometer's sensor has to be very close to the Hydra's sensor. I usually rest the known good Hygrometer right on top of the Hydra's display. Also, I found it is faster/easier to do if the Hydra's reservoir is removed, and trip point is reset super-low so the fan never comes on. YMMV.