This is a very useful document for calibration. it includes the hole range of humidity at a specefic temperature values with different salts.
Hope it's gonna help some people 
search for these links on google:
Salt tests:
http://www.omega.com/temperature/z/pdf/z103.pdf
how to calibrate:
https://www.omega.com/manuals/manualpdf/M969.pdf
To calibrate your sensors you should put them enclosed with salt at least 1 hours (until no change in reading values), then deduct many functions for each interval of humidity:
Yi = f(x) , i=1,....n
wich x is the wrong value given by the sensor, and Y is the supposedly correct humidity value to display.
then you add some conditional if statements in each interval to correct the humidity value.
I hope it was clear!!