I could not find details on the meter on Amazon, but I did find the manual in PDF.
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In terms of specs, it is your typical chinese amazon product. There is no detail. For example, accuracy should not be linear on a device like this. It should be less accurate on the edges of it's capabilities. I cannot find info on that or a chart to guide me like you could on say a milwalkee tools version.
Also there are wood and paper groups that you have to set the meter too based on what you are measuring.
I have no idea if dried cannabis is close to cardboard or basswood on their calibration scales.
Your 20% and 12% might be right, or it might be off.
Glad you got it figured out for this go, but going forward we might be able to calibrate this thing to give you reliable data if you want to jump thru some hoops. Your call, I'll help you if you decide you want to.