In the International Building Code you are allowed to have a riser height of 7″ for most occupancies.
However, it theater aisles you can get away with a lot more. Where the graident of the seating area matches the stair gradient you can go to an 8″ high riser allowing your stais to be 14% steeper.
If the seating areas need to be steeper for sightlines, you can go to a 9″ high riser, 28% steeper than a standard stair. With an 11″ deep tread and that’s a 9/11 slope or 39 degree slope.
