How can you tell it's at 13% humidity?
Pipe smoking is about relaxing and enjoying your smoke, not getting technical and calculating everything. You take a pinch of tobacco, squeeze it between your fingers, if they stick together then it is too moist, if they don't stick and don't have elasticity then they are too dry, it should be sponge-like, after squeezing it, the leaves should return to their previous states slowly. That pace will actually indicate how dry or moist the tobacco is.