Here’s one basic way of looking at it… Generally you want the peak force on the piston/crank to occur at 20deg ATDC. Ignition timing is one way of adjusting that. Of course, 20deg is a moving target because it changes with respect to time as the engine speed changes.
Up to a point, you’ll often want as much ignition advance as your setup will handle. Again, speaking very generally here. Factors that limit this are the fuel you’re using (it’s octane), compression ratio and squish area. When you hit the limit for timing… you’ll start to get pre ignition and/or detonation.
One thing to keep in mind for others reading this… Measurement of advance in distance varies with stroke and conrod length. That is to say, you can’t compare timing between engines of different stroke and/or rod length… At least not without a conversion. Other variables should be taken into consideration too, fuel, compression etc.
Timing is a pretty big and interesting topic in and of itself so I’ll break this one off.