My unit (a dehumidifier, given our climate in the upper left corner) is set to 50%. A seemingly reasonable balance between mildew control and dust.

But when I load sheet film I usually turn it off, then run some hot water into the sink to get it wet, then leave. After 30 minutes or so when I return, the humidity has bumped up and the air has settled sufficiently that I usually have few problems with dust on the film.