I know this thread is getting a little stale but thought I'd throw in my 10 cents worth :)
I tend to buy the Hygroset II, it is generally cheap and no less accurate than other more expensive ones.
Using a Boveda calibration pack, after ~12 (overnight) once it gets to between 73% - 77% I'll set it at that. The Hygroset II is accurate to +/- 2% RH, so with the Boveda bag at 75% it can be registering between 73 and 77, typically I can easily get it to 74 or 76. I'll then let it sit for the rest of the day and make sure it does not change, all being good I can comfortably recalibrate 1 hygrometer a day. As my cigar stock has grown I have bought bigger humidors but kept the old also, the cigars seem to always fill my available space, funny how that happens, isn't it.
I try to recalibrate every 3 - 6 months, I put a small self adhesive label on the back with the date so it is easy to tell when I need to.
I don't recall seeing any hygrometers state a greater accuracy than =/- 2% I think this a limitation of the technology and market forces, there's no motivation to make one more accurate.
Anyway, I don't sweat it, a few % here or there won't matter given the difference in opinion over what is the 'corect' humidity, just kepp it close and as stable as possible is best IMHO.