May I suggest that you look up the Google Sketchup tutorial on YouTube regarding array copying using the protractor tool. You can populate your space by number of seats(copy multiple) or by division of the arc. I do not know how large your auditorium is, but an effective design starts with optimized radiuses from the center of focus and careful consideration of lines of sight, as in sloping floors. In fact, major seating manufacturers use a variety of seat widths to optimize rows. If you are just doing somebody's home theater, you probably won't need this criteria.
I take the slow, deliberate approach in my aimless wandering.