RE: Noise measuring
If you follow the FAI advice suggested by Red B (and why not as it's the definitive competition method) and stay with that sound level you'll end up with a very quiet model.
Every F3A plane I've seen in competition has been VERY quite compared to even the quietest models used a club level. My planes often appear completely silent during vertical flight at the ends of the box. This can be somewhat concerning as the only indication that the engine is running is the exhaust smoke!
What you'll experience is that different propeller produce a great deal of difference in the noise level. Brand to brand and pitch to pitch.
Also a great deal of ambient noise that you hear is generated by the airframe and not the engine. Wheels, push rods rattling etc. beleive me, you'd be seriously surprized just how much you can quiet'en down a model by changing to a tight set of wheels!
Do what Reb B said. Don't experiment with other methods of measuring. That's the one.