In order to calculate the effective gain of the DaLite HighPower screen, I need to know what my off-axis angle is. I'm not sure exactly what off-axis refers to, but I guess the idea is that with the screen serving as a center point, I need to know what angle exists between my mounted projector lens running to the center of the screen. off the screen, and down to my eyes across the seating position.

Can someone tell me what the formula is for calculating this angle?

For one particular set of numbers, the projector lens will be 89" high off the floor, with a run of 15.5 feet to the center of the screen. The center of the screen is 61.5" off the floor. Then my seating distance is also 15.5 feet from the screen. With my eyes at 34" off the floor when seated.

Based on these numbers can someone please tell me a) what is the off-axis angle, and b) what is the formula you used to determine this. By having A it will enable me to calculate the screens gain at my seating position (I have a chart for this). By having B I can run various sitting/throw scenarios to see how I can minimize the angle.

Please note that in this case the projector throw and seating distance are identical at 15.5 feet so I guess this forms an isosceles triangle. However please do not count on this in your formula, as I want to try some scenarios where my throw distance and seating distance will not be the same.

Thank you very much in advance!