Depends on what you're most comfortable with. Figure out where you're most comfortable (probably around 40%) and go from there. Most of us are more comfortable with low humidity, but you don't want it too low or it really can dry you out. So do some experimenting and figure out what's most comfortable and use that.
I am living in the desert, very seldom do we have humidity, during our summers we have problems with the quality of the air that is the only time I have problems
I would say in the middle. High humidity bothers me and then when It is dry I have trouble breathing.