One the least helpful things to try to adhere to is THX dictates. They are NOT absolutes....just averaged out recommendations. THX cannot know the specifics you must deal with (Screen height limitations...chair dimensions...distance to Screen limits...etc.)
An important aspect to be considered is how tall the backs of the 1st Row seats are.
In your case situation, the highest you can go anyway is almost flush height with the Door Tract.
Take a measurement from the Ceiling down to the Bottom of the Tract, add the actual Screen's height to that, and what you have left is all you have to work with / around.
Just from what I already know / see you should be just fine with most seating upfront and the Rear seating up on a 10" Riser.
But as a working example, if the bottom of the Screen is at / above 25", you'd have no worries at all with the prescribed Riser Height.
20" or less and careful selection of seating as well as at least a 12" Riser is of paramount importance.
But nothing is better than hanging the Screen where it absolutely needs to be, placing a Chair at the front Row location, using a yard stick vertically on that back of the Chair Seat, with tape applied on the yard stick at the height that you can estimate where the Top of a Head would be (...or the top edge of the back of a "non-reclined" Theater Chair...) and the stepping behind the chair and from 5' behind, lowering your eyes until the bottom edge of the Screen recedes below the Taped mark. Note that specific height, subtract the height of the Chair, add on a couple inches, and you will have the needed height of the Riser.
Not nearly as complicated as it sounds.
An important aspect to be considered is how tall the backs of the 1st Row seats are.
In your case situation, the highest you can go anyway is almost flush height with the Door Tract.
Take a measurement from the Ceiling down to the Bottom of the Tract, add the actual Screen's height to that, and what you have left is all you have to work with / around.
Just from what I already know / see you should be just fine with most seating upfront and the Rear seating up on a 10" Riser.
But as a working example, if the bottom of the Screen is at / above 25", you'd have no worries at all with the prescribed Riser Height.
20" or less and careful selection of seating as well as at least a 12" Riser is of paramount importance.
But nothing is better than hanging the Screen where it absolutely needs to be, placing a Chair at the front Row location, using a yard stick vertically on that back of the Chair Seat, with tape applied on the yard stick at the height that you can estimate where the Top of a Head would be (...or the top edge of the back of a "non-reclined" Theater Chair...) and the stepping behind the chair and from 5' behind, lowering your eyes until the bottom edge of the Screen recedes below the Taped mark. Note that specific height, subtract the height of the Chair, add on a couple inches, and you will have the needed height of the Riser.
Not nearly as complicated as it sounds.
