I was going to say a ton of down elevator. Even my 33% Cap 232 will not flatten out on low rates.
I flip it to high rates (on all surfaces) when I want to do inverted flat spins. High rates is about 43 degrees up and down on my Cap.
As they have posted above, once the plane is coming down inverted play back and forth with both the throttle and the aileron to find where your particular plane likes to flat spin the best.
Dale