About 0.010" or 0.012" is pretty standard as the correct air gap for all of the common engines. Most people just use a visiting card (usually about 0.012"), because if you use a feeler gauge it will stick to the magnet unless you make sure you aren't on that part of the flywheel.
I recommend that you never hold the end of a plug lead and turn the flywheel unless you turn it very, very slowly with your fingertips to check ignition timing. Holding a plug lead when the starter is being pulled is likely to do you harm - at the very least it will spoil your day. It creates a current path from one hand to the other, right across your heart muscle. You are applying your own defibrillator just when you don't need one. Fortunately with a small magneto the energy discharge should be lower than a real defibrillator, but it is a very dangerous thing to do, and pointless as well. The purpose of the ignition system is to create a spark, not to electrocute the mechanic, so the only useful test is whether it creates a fat blue spark.
