RE: Home made control horns
Doesn't the EPA only affect the end points? That would still leave a bit of differential in the response prior to the end points being reached.
For myself I'd just skew the output arm or wheel to compensate. One or two adjustment notches angled to the rear of the model should match the angle up closely enough. I'm with Darren about the "close enough" but it's so easy to just put the arm/wheel on at roughly the same angle that it's not a big deal.