On my AeroWorks Extra 260 I have 45 degs of elevator travel and 50 degs of rudder. It takes all of both inputs and a little opposite aileron with the throttle slightly higher than idle to get a decent flat spin. My CG is also far enough aft that the plane needs NO stick input to maintain level flight while inverted. It tucks slightly to the gear on knife edge so it's quite tail heavy but it does beautiful waterfalls, knife edge spins, etc. It does awesome elevators, harriers, and also hovers quite nice. Point of all of this is the CG needs to be pretty far back with a bunch of control travel to get a good flat spin. The ratio of elevator area to horizontal stab surface area is important too as while in the spin the relative wind will tend to push up on the stab and attempt to push the nose down causing it to break out of the spin. This is why some planes flat spin well and others don't.