I don't rely on a tachometer too often to tune (mixture) my engines.  I usually try to adjust the needle to where I think the exhaust note -sounds -about right.  Fly the plane and make more adjustments if necessary.  After awhile, you can recognize the exhaust sound that produces the best engine run.

That being said, I have checked rpms and on the LA.40 engines I have, 10,000 rpms + or - a few seems about right.  I use APC 10.5X4.5 props on most of the .40s.

I'm sure others have differant methods that work for them.

Joe

Dwayne,
It makes some difference whether this is a profile or built up airframe. profiles tend IMHO to pick up more rpm as you launch as a result of the head pressure in the tank changing, because the pickup tube is farther outside the needle valve than on a built up.  That said, My LAs launch on my profiles at about 9800 with a 3.75 pitch prop and a .282 venturi, Note however this is a .46