Anywhere between the spar and leading edge should be safe territory for balance. If you are a rank beginner, leave the muffler on and add a bit of nose weight. If you can competently fly the model without crashing and do simple tricks, try it where it is after a few flight with some extra nose weight.  Doc example is the old school method like he said, but it works, just keep any weight additions or removals a little at a time.
  Type at you later,
   Dan McEntee

What Doc said if you're learning.  If you really want to trim for performance then you want to get a handle with adjustable leadout spacing -- then you adjust the CG to get the plane to do the right things, and you adjust your handle so that you get the right amount of turn when you ask for it.

   Might run better ad ne a bit more easy to tune if you swapped the needle valve assembly around, put the needle at the top. Will shorten the fuel line run and be lots easier to adjust the needle. You can shorten the spring a bit also, just unscrew the knob, cut the spring in hall, screw the knob back on, put a touch of CA on it to help hold it.. You will notice that it doesn't shake like a tuning fork that way, and that can affect the needle setting and how the engine runs. If you see the spring shaking all over the place, the pointy end can also. They are connected.
   Type at you later,
    Dan McEntee