How well the arc length method works depends on how large a diameter you have to work with. If you do it on a large flywheel, then precision is not a problem down to 1deg. Off a smaller prop hub, though, you might at best be able to eyeball it between the typical range of 28-32degs.

The degree wheel amplifies the resolution and is particularly useful when you are power-timing the engine, where you may be advancing the spark 1deg at a time.