I've found 60% to work fine. All of my guns are exposed on the wall, and it's sad, but some go several months without a wipe down. I've always had a room dehumidifier and I can count on one hand the number of times in 30 years I've found a spec of rust. I believe 50% is absolutely critical for the oil finishing I do and when necessary I can get there in 12 hours or so. I see no reason to go any lower.