Ok, here is what you need to know. First of all you need to know how many cubic feet there are in your grow room. So just as an example if your room is 6 ft high, 6 ft wide and 6 ft long, you have 6 x 6 x 6 = 216 cu. ft.
Have the room closed and in this case your MINIMUM fan INTAKE should come in from near the bottom (floor - where temps are coolest) of the room, and should be at least 400 CFM (cubic feet per minute). Then you should have an opening near the top (that is near the ceiling) that should be at least 216 CFM (or 200 CFM) as an exhaust. That way you will have positive air pressure in your grow room. That means no odors should be leaving your grow room - normally an air scrubber would be placed on the exhaust and cleaned so only the warm clean air is being released to your neighbors. You should also have some fans just moving the air around inside your space.

A 400 Watt lamp only puts out about 1500 BTUs, and there really shouldn't really be a 'heat' problem. 18" away from the top of your plants is too far, wey too far. I have a 600 watt, and its generally only 3 or 4 inches from the top of the plant (I have vented lights). If it was me, I would never place a thermostat right above the plant and check the temperature there. All you really need to do is hold your hand there. If its comfortable for your hand it will be ok for your girls. The ideal temp for your room (not too near a light, and maybe half way up (4 ft from the floor), is 75F, and during lights off the temp should be around 65F.

I've worked in the Heating, Ventilation and Air Conditioning trade for about 10 years.

sn