Try the small
Boveda humidification packets. I've used them for years in my cigar humidor, and now use the m in the square tins. If you enjoy the aesthetic of the tin, then this is a great way to keep the contents at the moisture content that you desire. The packets control humidity both ways: They will absorb moisture if the surrounding humidity is above their rating, and they will expel moisture is the surrounding humidity is below their rating. That means that when they are rechargeable. When they start getting low on water, just put them into a sealed container with a damp sponge and they will reabsorb the water and be good as new. Hint: Don't let them dry out completely, that is usually the death knell for them, and keep them out of direct contact with the damp sponge.
So what is the correct humidity? That's kind of trial end error depending on the tobacco and personal preferences. Here's a chart, derived from data collected by Greg Pease:
I find that the 62% packets keep most tobaccos a bit too dry. The 65% are good, but do not come in the small size. I use the large packets in jarred tobaccos that are either too moist or that get opened frequently, but they will fit into some tins. The 69% packets are great for tins.
Amazon has the best pricing that I've found. Just be sure to double-check which size that you are buying, it is easy to buy the wrong size.