Yep getting a decent hygrometer can be a long walk on a short pier with a lot of wasted money and effort but you can benefit from not making the same mistakes that some of the rest of us have made.
Edward I used a wet bulb test which is really the only way to accurately determine your current relative humidity to check some digital hygrometers and also to calibrate my Abbeon hygrometer. Wet bulb tests can be done with two like thermometers or a sling psychrometer, same idea.
I found my Caliber III's to be about 9 points off and more importantly in the range that we need to see RH wise, about 42 - 48%. What's interesting to note here is that if you test for accuracy say in the 50 - 60% range the amount that these digitals can be off can change. So checking the calibration of any hygrometer should be done in the range that you intend to maintain and not at say 75% with a salt test.
Waddy is correct that the instructions that came with the Caliber III indicate that a salt test can damage the unit. I am not sure if this is true or just a defensive measure to help prevent anyone from determining that these things are not much more accurate than a counting horse... with no shill to tell them when to stop...
What I use here works great for me but can be a little pricey. I have a Abbeon hygrometer that I used to send back to Abbeon every so often for calibration, they offer certified calibration services for their products.
Now I simply do a wet bulb test every 3 months, compare the results in the desired range that I maintain my shop to the Abbeon and calibrate the Abbeon as required. So far my Abbeon has not been more than 2% in a 3 month period which is pretty good.
Once you have a way of actually knowing what the RH really is you can use the digital and note on them how far that they are off in the range that you care about. For example I know that my Caliber III's are off 9% based on several tests that I did. So I use the Caliber III's anyway but just add 9%. Same holds true for the Stew-Mac white and blue hygrometer that I tested at being 11% off in our target range.
My Abbeon is my "at-a-glance" go-to hygrometer though and again every so often I check it with a wet bulb test.
I hope that this helps.