There's a sensible discussion of obtaining absolute humidity figures from relative humidity numbers in...
https://www.sparkfun.com/datasheets/Sensors/SHT1x_datasheet.pdfI say "sensible", because it goes some way towards explaining "the right" answer, but ALSO gives a less complex route to a "good enough" answer.
Are you sure you really WANT the absolute humidity?
I believe that if you are worried about things in the attic being damaged by dampness, relative humidty is what you want... but not sure about this. Other people's thoughts?
I believe that people who store rare books want to achieve a steady temperature AND steady RELATIVE humidity. Of course, if they achieve the former, the differences in the latter become less complex.