I would determine the typical SPL and factor in some headroom in the room before anything else- the amount of absorption will depend on this because a live room won't be terrible at low SPL unless the sound has a lot of sharp transients like gunshots, drums, etc. At High SPL, first reflections won't be enough, but distance between surfaces can be your friend and if the speakers are far from the side walls, you may be able to prevent some of the mid-range flutter, but the low frequency could be trouble unless something can be placed in the corners to break up the energy.

I was surprised by the small theater I recently finished in the ballroom- while not what I would consider 'great', it was still good at -10dB on the AVR-X6200 volume scale and the SPL was high enough that it shouldn't seem to be inadequate for a relatively realistic movie experience.

I don't know if you have used Room EQ Wizard, but one of the new tools is a room simulator that lets the user set the room dimensions, place main & surround speakers and multiple subwoofers, in three dimensions- this is great because the height makes a big difference in the predicted response and it shows notches from room modes. It also works well for RTA and it's what I used when placing the panels in my room when the sound was pissing me off. I had a deep notch in the usual area and it's now gone. The response is very smooth and the only reflections that annoy me would be hard to treat because they're coming from the side of a kitchen cabinet.