Having had to sit through 8 hours of FIA zoom meeting last Wednesday, I managed to retain a few bits, and lo and behold maybe relevant to this Marek.
The optimum angle between back and base (safety and comfort) has been tested at 105 degrees opening angle. Equally important was that the base angle is not more than 10 degrees from horizontal. Given that the cushion you have in your possession was designed to that Marek, flat on the floor would meet those numbers!