If the flywheel key is OK the ignition timing can't be all that far out. If you have electronic ignition plus the key is OK, it can't be out at all. Personally I think I'd rather use an ordinary automotive timing light so I can check everything at once, but then the first time you do it you have to mark top dead center on the outside of the flywheel. I just hate taking the flywheel retaining nut off to get a look at the key - and anyway I got in the habit of using a timing light.
The flywheel is in a fixed position providing the key is not damaged at all. Most engines these days have a fixed position for the flywheel and the key is alloy which sometimes gets damaged slightly and throws the timing out.
If you were working on an old villiers it is a different story where you need the top dead centre setting for the timing as the flywheel is on a tapper.
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