Screen sizing - conflicting advice

I'm getting some conflicting advice from " the experts' and am soliciting Agon advice. I've purchased a used Digital Projection 3 Chip DLP Lightning 15sx (MSRP >$110,000)for a little bit more than I can buy a Seleco HT 300 plus for!

It is an amazing machine to say the least - 12,000 ansi lumens (No kidding)! Fortunately you can adjust light output down. In talking with Stewart and DP, they both agree a GreyHawk negative gain screen is necessary, however, size is the issue.

My theater is 20'Long x 16' Wide and 8' High. I want to have 2 rows of seating and use a 16x9 screen format. The projector will be mounted behind the rear wall of the theater.

Stewart says first row of seating should be roughly 5 times screen height for 480p output. On a 16x9 120" diagonal screen which has a 53" screen height and 96" width, that would put the first row at ~22'.

DP says 1.5 times screen width is sufficient, which would put the first row at 12'. DP says that you should get a s much width as possible because it is screen height that causes fatigue, not width. They feel that more than 80% of the action is in the middle of the screen and the sides are more filler than anything else.

I'm sure I'm missing something here. Can you help?

Thanks.