FWIW, my AC Infinity is the closest hydrometer I've seen, it was off about 1%. I used 62% Boveda packs as they are closer to target humidity and I didn't get good results with the salt.Good idea, I've had two AC probes. The first one was off by 10-12 RH points, the replacement is within 5 or so (which can be compensated in the firmware).
You can calibrate one or both with a table salt slurry and a ziplock; it should read 75% after a few hours.