Actually, you can calibrate that thing with a Boveda bag. It has a screw adjustment on the back of it.
Remove the hygrometer and put it in a heavy duty double ziplock freezer bag with a fresh Boveda bag, leave on counter for 24 hours.
Read the hygrometer reading through the bag without opening it. It should match the Boveda bag. If not, mark the offset. For example, if you have it in a bag with a 65% Boveda bag, but it reads 68% after 24 hours, you write down the hygrometer has a +3 offset.
Remove it, use a small screwdriver to bring the needle down a minus -3 degrees. Put it back in the back and seal it with the Boveda bag and check again in 24 hours at which point it should agree with the bag % RH. If still off, readjust.
Now you will have this thing calibrated in the range of the Boveda bag, and then you should be confident in it. But I still prefer those electronic ones. You can calibrate them the same way.
JC