First of all, welcome to BOTL Curmudgeon76. Sorry it took so long for me to respond. Work has been crazy. I can hardly keep up with reading posts these days much less writing some. If this continues Ill be forced to hit the lottery!
Anyway,
I have the Cal III hygrometer and yes you can check it with the Boveda test kit. I spoke with the people at Western Humidor and they told me that the Boveda test kit is what they recommend if you want to see where your hygro is reading at. I agree with Jwrussell about people spilling salt water on the hygrometer. That will surely ruin the sensor. The first one I had read 75%. The Boveda test kits are supposed to be 75.5% (with a +/- 0.5% tolerance). The Cal III doesnt read to tenths so this was more than adequate being in the +/-1% range. If the hygro reads further out by 2 or 3% you can just do the math or call them, tell them its out of spec and they will replace it for you. The Cal III has a 5 year warranty, whereas most Ive seen in that price range only offer 1 year.
I read somewhere that all digital hygrometers can drift over time and it was recommended to check them once a year. I had my first one well over a year when I re-tested (with a new test kit) and sure enough it had drifted up 4% to 79%. I called Western Humidor, explained the problem and they replaced it with another unit no questions asked.
Some of my friends have the Cal III, multiples in fact. Some were off right out of the box as much as 4-5% and others right on the money and are still reading accurately. Luck of the draw I guess but if you have a problem theyll take care of you.
If you buy it from JR ($19.95) and its not to spec., you can return it and theyll pay the return shipping and send another if you want.
Hope this helps some.