John,
I've got a room just about like yours, except the ceiling is higher. Calibrating to 75db is more than sufficient. If you don't calibrate the system per the manual, you will most likely be disappointed in what you hear and you're not going to get the true intention of the soundtracks of the DVDs you are watching. The Radio Shack sound meter is reasonably inexpensive, well worth the investment and takes all the guess work out of it. Just make sure, as indicated in previous posts, that you start with the volume at 0db when you begin your calibrations.