You can run a "wet" nitrous kit with propane just like you would with gas. But instead of getting fuel off the fule rail you would do just like you did. Plumb the fuel noid up to the propane bottle. Its becoming more and more popular.

I think you were on the right track with a few exceptions.
#1...you should have gone to a dyno that had a wideband 02 sensor so you could read your correct AF.
#2 you can tune your AF by using the proper size jet setup. You were correct to go rich at first. But you should have scaled back the size of the propane (fuel) jet as needed till you got the desired Af ratio of...say....12.0 to1. The fuel/nitrous jets called out for in the instruction manual will not be accurate when using propane.

From there you could have extrapolated the correct fuel and nitrous jets for what size HP shots.

All in all this is an EXCELLENT way to boost HP on cars that dont have any room for extra HP due to a weak fuel system. Also with the high octane you will be able to spray more hp before detonation occurs or timing retard is needed.