Like the second suggestion.
if there's enough degree range of movement on the Mag fine-tuning on the slots on the roadside...bit more me that.
You should have enough movement to see which way works best, i.e. more or less advance. I f you find that you get to the end of the slots and still improving then you cand remove the magneto gear, move the magneto in the direction you were finding best and test again.
In most cases, I wouldn't have thought that you would need that much adjustment but as Davidd points out this could be dependant on your engine specification. e.g. Davidd may have been using twin plugs.
Be consistent, e.g. find TDC the same way each time, preferably using a stop through the plug hole and a degree disc, turn the engine in the same direction up to the stop point created by the timing rod in the magneto and don't alter anything else, e.g. jetting as you have introduce another variable and will not know which one made it better/worse.
Make a note of what you've done for reference later.
If you are able to check your timing with a strobe it makes it easier to know where you are from a firing degree point of view. Take your reading once the degree disc shows that the magneto has stopped advancing.