Sorry I have to fully disagree, tuning to a set RPM plain makes no sense. Every bar and chain is a little different, be it 16 inch 20 or 25 it puts a different WOT load on the saw and that affects the RPM. Prove it to yourself, put a 16 inch bar and chain on, better yet make it a .325 or a lightened race chain then put a 20 inch bar on with 3/8 sprocket, then throw on a three footer with some .404 take note of the WOT RPM each time, don't touch the mixture.
Get the filter a little dusty or some carbon on the exhaust and the saw will turn a different RPM even if it was possible to give it the same air fuel mixture. Then no two saws are the same even same make and model will have slight variations. Keeping to factory spec with a aboslutly stock saw in A1 condition and you will be in the ball park for mixture. But that does not the ideal mixture.
If you learn to tune by ear and read the plug and do some homework on cut times you will do just fine and will consistently cut faster than someone who just sets it and forgets it. Where a tach comes in handy is once you know where a saw cuts best with the set up you use, then it's easy to keep track of it and set it to a known point.
Factory RPM applies only to stock saws in good condition and it is a conservative simplification. Likely done to keep dealer techs from blowing saws up.
If you hand a customer who knows nothing about tuning a saw one day with one bar and chain you would want to be sure it's still ok the next day with different bar and chain, fuel... ect. Thats where a conservative factory spec works on a stock saw. It's all fine and good if your not dealing with a modified saw or really don't care that much about performance.