I know you should store ammo "cool and dry", but does anyone know the relative humidity below which ammo should be kept for long term storage? Is humidity above say 50%, or 60% bad over time?

I know I'm probably being too anal, but I have a couple hygrometers that I use for cigar storage (where humidity is VERY important), and wanted to assess my ammo storage conditions. I'm also considering storing my ammo in my basement which is a little more humid than the rest of the house (although not damp).

Yes, I do own sealed ammo cans and have some desiccant on the way.

Thanks in advance....

Lower humidity is better.  You want to avoid the possibility of either the brass in the case or the gilding metal in the bullet jacket corroding, so you want to reduce any potential electrolyte to prevent galvanic or chemical corrosion.  Some people are concerned about the possibility of "drying out the powder."  I don't think that's a real issue with well-made ammunition; just the metal-to-metal seal should keep the case well sealed, and I doubt you can hurt smokeless powder by "drying it out."