They reckon 3 degree trim is the best, they being the guys on boatracingfacts. Ideally your trim would be when the centre line of the prop is parallel to the water surface, as this is when you're prop is delivering it's maximum forward thrust. Unless you have a highly optimized hull which developes exactly the right amount of lift at a certain speed you'll need to trim your engine to compensate for the lack of/too much hull lift.
With thundercats, I find that at speeds upto 50 knots its quite stable with about neutral trim. Anywhere over this and you start having to tuck the engine in as the amount of air flowing between the hulls generates too much lift. This is why the power required to achieve higher speeds is exponential, you're basically fighting the lift generated by the hull by using the engine trim to push the bow back down.
Without re-designing your hull to suit your setup specifically, the best you can do is trial and error
. If you're chasing numbers props are the way to go, get yourself a tacho and a GPS