Head on, head off what do you use to get TDC

Ta.

If the specified advance for a commando on petrol is 29 degrees, and you get it wrong by two degrees either way - if you then re-jet to the timing doesn't that give substantially the same result ? I usually set the ignition advance, then jet the carburation. I don't like re-setting the timing without checking the jetting to get the best to suit it. From my experience, particularly with two strokes, the situation is very sensitive to ignition timing changes. And if you are running your motor close to the jetting limit a small error in timing can do damage, even in a four stroke engine. Changing the exhaust back-pressure without re-jetting is also not good e.g. if you fit a different muffler (megaphones are deadly).