Searched, no results.  I had a room built in the new house, downstairs, 13" thick concrete walls, painted with that Dry Lock paint.  Dehumidifier has gotten humidity down to 38%.  Where should it be?  Would golden rods help.  Room is about 12'X12' with a 12' ceiling.

     Don't get too hung up on numbers . My gun cabinet is in a walk in closet just off of my
master bath . Probably the worst place to put it in my house , but I didn't have any other options .
I make sure every gun is wiped down before it goes in the cabinet .  I haven't had ANY problems
with rust .  Do what you can do keep the room dry , keep your guns oiled and free of fingerprints, and you should be OK .

I treated every weapon before I put them in there.  I just don't want to pull my Colt Gold Cup, NIB, 70 series and find a little rust.   Rust on an Uzi, Wilson Combat Tactical, FAL, AR50, Weatherby, Walther P88 or Zeiss scope would be worth the cost of preventing it.