A. My philosophy is that if you know the actual RH by "doing the math," then I see no reason to invest in a new unit, assuming it's analog and you want to go digital. I have a digital unit that's off by 10 points. So if it reads 60%, I know it's reallly 70%.
If your hygrometer is analog and you can calibrate it, when you're done with the salt test, depending on where the needle is, move it to 75%. It takes about 6 hours to do the test right. You may find it a little more accurate. IOW, you won't have as wide a gap when you read it.
Some manufacturers now make a digital hygrometer that you can calibrate., so you might want to look into one of those. | 
