What humidity levels do you keep in your terrariums?

We recently had a fairly cold period here and that really messed up the humidity levels in my terrarium, forcing me to mist just to keep the relative humidity above 40%. That part is slowly getting under control, though, as I'm redesigning the ventilation in my terrarium. What pisses me off is how I didn't see this coming when I made the damn thing, and now one of my snakes just had a failed shed because of it.