No, your wide open throttle rpms with no muffler gives you a baseline. Then you install the tuned pipe long, and start shortening it a little (about a quarter inch) at a time. When the rpms stop increasing, make the setup about 1/4" longer and fly it. The rpms you end up with over and above what it did without a muffler is an indication of the increased power the pipe is providing. (and lets you know it's working!)

AV8TOR