50% Humidity is fine, mine is usually around 50-55 as well. I've hard that at 60% and higher it will actually make your carbon filter less efficient to the point where it won't work very well at all.
My plants grew just fine in 50% humidity. Also 85 is not super high, as long as you keep the tempurature from fluctuation more than 20 degrees, it should be okay. Mine got into the High 80's at the hottest parts of the day and they still did fine.
I would reccomend running the ventilation and not worrying about the humidity too much. Just make sure you plants have nice moist soil and that they don't dry out too much. And if you like, mist your leaves lightly before your lights turn off, they'll love it. Just don't mist with lights on and if you have to make sure the droplets are tiny, not large enough to cause burning from the magnification of the drop.