Hi, I don't think you'll increase the humidity too much if your tank is well ventilated. To keep the humidity higher you have to reduce the amount of ventilation until you reach the desired humidity level. Also, you may want to increase the frequency with which you're spraying and reduce the time of actual spraying. ie. spray for 5 sec every 30 min. Best achieved with a repeat cycle timer. Not sure if you're using it or not.
Read the timer recommendation on MistKing if you don't have one:
http://www.mistking.com/faq.html