Unless there is a lot of headspace, tobacco will not lose moisture in a Ball jar. If the tobacco is already too wet, or too dry, I put a Boveda packet in, and leave headspace.
I find that almost all tobacco ships too wet. They ship wet for a mumber of reasons, but the primary reason is that tobacco is processed wet. Dry tobacco will crumble in process and handling. When I order more than 2 oz or so, I split it into 2 jars: One to smoke, and that gets a 62% or 65% Boveda, and the other is left as is until I need to restock the jar in the ready position.