Best advice I can give you is don't waste your time. If you want altitude hold, the best solution is to just go buy a TruTrak ALTRAK unit or Trio EZ1.
These types systems have not performed well in RV's or other high speed aircraft at all. The trim tab is always behind the control surface movement, which ends up leaving your plane "hunting" a bit.
These have been discussed to death before (perhaps on the RV list instead of here...I don't rightly remember) and tried without much success in RV's.
Just my 2 cents as usual!
Flying behind an Altrak and loving it!