I have a constant height setup. First row is about 13ft back, second row about 18ft or so. For material using my entire screen 2.35 (130" diag.) the back row is best, however, the front row is ok if your not watching a lot of fast paced action movies. Otherwise, your eyes dart back/forth a lot.
For 16:9 HD or 1.85 ratio movies either is fine and 18-19ft is not to far back. The diagonal then is about 105".
A good rule of thumb is 1.3 to 1.5 times the screen width for the first row. However, a lot depends on the type of projector your using. You should not have a problem with the latest technologies.
Randy
