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