That's normal that the measurements are of on those cheap little things. What I do is the following. I activate all of them at the same time.
And put them in a big jar with inside a sloppy solution of table salt. At 20°C there should be 75% humidy inside the jar.
Not the difference and write it on the casing of the device. ie, you measure 73% so I would write +2. That way I know that any value coming of this device needs 2% added to that.
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Now I hear some of you thinking... That humidity log isn't linear and that's right, but this will be close enough for our purposes