Help?! Temperature + Humidity + Light Distance = Triangle of death

Hey guys

So my grow is going well... 600W MH vegging away (1 month now approx), 5-7" plants, 5gal pots, light is 18-20" above the plants. Thing is though, it's hot and dry. My grow room is in the basement, with usually 65% RH. Now it's 40% max, and i'm getting 85F with a table fan blowing on my ladies 6' away. I rerouted the hvac in my home to air-cool the light. Ok, temps dropped about 5deg. When I turn on the AC, I get another 5 degrees off. We're talking major AC here, the setup is for the entire house, but it's now rerouted to my grow light exclusively. So ok, 75F and 35% RH with AC on. Too dry. And the rest of my basement is too cold.

How do you guys get your light(s!) down to 12" and not kill your plants??? Should I get a floor-type AC unit and a humidifier for the room and seal it up? (It's currently an unfinished room, so air escapes from the missing ceiling).

I measure my temp and rh at plant height, which is the way to go, right? And I did the "top-side of your hand not burning = ok for your plants" assessment and frankly, 85F and I don't feel the slightest burn. It's warm, but nothing more.


Suggestions?

I'm not aiming for 1st place in crop size. I just want to grow my thang, without too many problems.

thanks

Hmm you can get a humidifier for the room or I use a swamp cooler if you can find one circulation is the key imo my room goes up to 90s some days when I work long shifts and the tops of the plant are about 5in away and never burn good luck to ya

Evaporating water machine, perfect

I modded another air duct just now, so now i've got cool air to my light, and cool air to my room.
It works so well that the metal reflector is room temp
add that swamp cooler and we're in the sweet zones. Lower light, finally!
thanks