All good questions. To time the ignition properly you do need to use a timing disc and a strobe light. That is a bit awkward on an Ariel. The good news is that you only need to do it once and you don't need to alter the timing by moving the magneto drive sprocket.
But, as it happens, I have another method. It is so easy to change the timing that you can do it very easily at the roadside in just a minute. You can time it roughly by using the mark that the supplier kindly put on the trigger unit. It is close enough to get the bike going.
Then what I did was to ride it. What I look for is for the pulling power to increase slightly when you back off the ignition timing from full advance but then to decrease slightly when you back it off a little further. I do it with the bike pulling up a slight incline. If it doesn't do that then it needs altering slightly. It's very easy to do in tiny increments.
In fact, since the trigger mechanism is completely waterproof, you can leave the cover off to make it even easier to adjust until you get it right.
In the end, that method was somewhat faster and rather less frustrating than doing it by the recommended method. It is also probably more accurate since it relates to actual performance rather than a theoretically "correct" angle.