What is the ratio of your transmission on the diesel? As long as the propeller speed at governor of both setups is the same, it will be close assuming it was setup right in the first place. The test that mitiempo proposed is also a good way to get a rough idea.
There is no such thing as perfect prop pitch without a controllable pitch propeller because you would ideally change pitch based on sea and wind conditions. As long as you can hit governor and you are doing hull speed, you are in the right ball park although it would be nice to do better than that.