Here is a link to a calculator I found in a thread on another forum.
http://www.cinegi.com/cgi-bin/riser.cgi
When using this tool make sure you realize that the front row height is to the top of the head and the back row height is to eye level. Assuming the seats are the same size the back row measurements should be about 5" shorter that the front.
Some other comments from that same thread -
A good rule of thumb is that you eyeballs should be a 1/3 of the way up from the bottom of the screen. This is just a guide based on a comfortable angle to view a movie. Some people may like the screen a little higher and lean their seat back, but most people don't want the movie fully reclined.
Some ways to improve Line Of Sight (LOS) in low ceiling applications:
1) Choose taller seats in the rear row(s)
Continental is one company that makes a "tall" chair to match a popular model that is like 5-6" higher than the matching regular height model that we would use in the front. This way the seated height is higher, but the riser height and standing headroom are still acceptable.
2) Make the front row the "money" seats, and choose non-reclining chairs for the riser that can be placed closer to the front row.
Irwin and a few others make nice non-recline chairs that are exact replicas of commercial theater chairs in the industrial upholstery or finer finishes. If you match the upholstery these chairs do not look out of place, and kids love them.
3) Raise the height of the screen a few more inches
If the rear row are your money seats, you will want to LOS to be as close to perfect as possible. You don't want the front rows to be craning their necks up either, nobody wants a stiff neck after a movie. Be careful how much you modify this dimension.
Depending on ceiling height, basement theaters will get usually get a 5-8" step up riser or a 10-15" two step riser, and even with the creative planning for the seating and screen height, the LOS is sometimes slightly compromised.