Sorry to hear that, wax. It has become clear to me that knowing the humidity of the garden in various places and controlling VPD has allowed the next level to happen. Accurate instruments are just too essential to cheap out on.
i still use the cheap ones, but you have to know how they read before using them. i put them into a jar with a boveda 62 and then see where they're at a day later. then i mark them so i know what to add or subtract to have an accurate reading. if after a day one reads 60%, i mark it with a (+2) so i know to add that on when reading it. i'm sure it's far from a perfect science but it seems to work for cheap hygrometers.
I have a bunch of the small hygrometers they will vary from each other quite a bit. When i get ready to use them i will wrap them up in pretty damp hand towel then put the towel in a 1 gallon plastic zip lock bag. Leave them in there about a hour. When you unwrap they should all be reading 99%. They should stay strait for a while. I might be wrong about this but i think that's the only way you can calibrate a digital hygrometer. Mine seem to work good doing this. I have never rely done any kind of testing to see how long they stay calibrated.
Listened to a podcast today that explained a measurement of “water activy” used in the beef jerky industry. Some braniac cannabis scientists applied this to the drying process using the same test equipment, and has found a QUANTIFIABLE number of water acitvity level that tells us exactly when the plant is at its perfect dry level. They also found all buds large and small become homogenous in terms of water activity once placed in a jar, and the internal humidity depended almost completely on the external environment the jars were placed in.