Unfortunately, this isn't quite as easy a question as it may seem, as different tobaccos have different hygroscopic natures. However, you can get pretty close. An RH of between 61% and 64% will equillibrate most tobaccos to the 12-14% moisture range. This may be too moist for some smokers. To get to an average moisture content of 10%, you'd need an RH of about 56%, according to my experiments. Of course, if there are humectants used in the tobacco, these figures will not apply.
For packaging and long term storage, a moisture content of between 13-15% is ideal. Most smokers also find this to be a good moisture level for smoking. Personally, I prefer it at about 11-12%, sometimes even 10%, depending on the tobacco. (Virginias require more moisture to deliver their full flavour than Latakia blends do. The moisture helps solvate some of the sugars, delivering them in the water vapour/smoke stream, rather than simply burning them, which will tend to produce a sharper, tangier taste from the acidification of the smoke.) By the time it's down to 10% moisture, the tobacco will be rather dry; below this level, it quickly becomes friable.
It's essential to keep tobacco moist during packaging and storage. Biologically, some moisture is necessary for the aging process to occur. Mechanically, the moisture keeps it from turning to dust when it's blended, packaged, shipped. Too, given that it's much easier, in most climates, to dry tobacco than to rehumidify it, we're always better off with tobacco that's slightly on the moist side, rather than too dry.
Your fingers are actually a pretty good gauge of a tobacco's moisture content. If the stuff crumbles, it's below 10%. If it sticks together in a ball, it's over 14%. If it's springy and malleable, bouncing back when compressed, it's in the right range of 12-13%. It's remarkable how a little difference in moisture content can make such a big difference in the way the leaf feels.
