I've learned more about hygrometers in the past couple days than I ever wanted to. I do track VPD but more importantly I want to know what my RH is. I didnt. Not all hygrometers are created equal and I had a half dozen brands mounted and or hanging all over the place. I put them all in the same place as an experiment and found up to 12% variation.
I'm putting this out there because the farm has helped me a ton over the years and I bet I'm not the only one dealing with this. Do your own research but a brief summary of what I've learned would be:
1. Some hygrometers are better than others. Even high quality units have a accuracy of +/- 5%.
2. They all need to be checked for accuracy from time to time.
3. Going forward I will be spending a little extra for units that can be calibrated. Boveda makes a calibration kit but you can google how to do a very simple and accurate calibration with a bottle cap of salt and a ziploc bag.