Mak,
I'd support Genghis' view that you don't have adequate data to do the analysis properly. You say that you only want an approx number so I wonder about the purpose of the test. If it is a 182 and the engine is working properly then it will perform like a 182. I'd concentrate on whether the airframe has had any major repairs, rigging and the condition of the engine.
FAA AC23-8 Appendix 2, in part 2 has the method for you. If you really want to try your hand at number-crunching, have a look at this:
http://www.avweb.com/news/airman/182418-1.html