Especially since you already have your seating, I would strongly suggest building a temporary platform to put your back row of seating on and make sure that the back row is not obstructed at all (by either the projector or the front row of seating/viewers).
In my basement theater, I had a few limiting factors like a soffit in the front that limited the screen height and a 7'8" ceiling (before furring and drywall) that limited the riser height. I ended up with a 12" riser after carefully measuring sight lines from the back row of seating.





9 months of work have lead to the creation of the man cave. Phase one will be completed tonight with carpeting the 2nd row riser & the hanging of the 58” plasma and home theater speakers.

i read all the post here. it make useful to me. same problem here. thanks!