Well, lets see...
1) Largest minimum operating RH for a communications device that
I happen to have the specs for is 20%.
2) Smallest maximum operating RH for a communications device that
I happen to have the specs for is 80%.
3) Static electricity is minimized when RH is at or above 35%.
4) RH below 25% can cause embrittlement of hygroscopic materials
such as paper.
5) RH above 65% can cause mold growth and metal corrosion.
6) Humans are most comfortable when the RH is between 20% and 60%.
7) RH above 50% in cold weather can cause condensation inside of outer
walls.
So... perhaps a target humidity somewhere between 35% and 45% would
be ideal.
-Richard