High Humidity

Hi i'm growing in a 4x4x6 grow tent with 400watt hps and my girls are 5 weeks into flower my humidity is between 70- 80 and when I leave the tent open it drops down to 60. What's the humidity should be at and what can I do to bring down the humidity besides using dehumidifier?

I try to keep my humidity down to 30-40 during flowering