now I just need to know how high the knee wall is allowed to be.
I couldn't find any limitation on that. Accordingly, the knee wall height would only be "limited" by the ridge height.
Let's assume the following values: raw floor level on the ground floor at "0.00", story height ground floor 285 cm, floor structure in the attic 16 cm (from which I assume you want to measure the knee wall height), then the finished floor in the attic is at "3.01".
Now we add your desired knee wall of 2.00 m to that: result: "5.01". At 46° roof pitch you would still be within the limit with a ridge height of "9.395". In practice, you will be well below that since you still have the full story behind you.
Conclusion: relax. The avoidance of a full story here sets relatively tighter limits than the absolute height. For example, at 22 / 25 / 28° roof pitch, with a 2.00 m knee wall, you reach a room height of 2.30 m already after 74 / 64 / 56 cm from the eaves wall – that is significantly too early. So, I see the candidate for your calculation probe, for example, at 25° roof pitch and 1.80 m knee wall.