Sometimes when we try to explain things, it just doesn't sink in. For example, I have and still have a hard time understanding just how you can determined TDC using a piston stop. I must have a mental block trying to follow the logic to a piston stop.

Maybe I can offer a different way to check timing that may be easier to understand than the usual methods. I have to make an assumption that with a typical sensor the spark will occur just about when the magnet is half way outside the left edge of the sensor (rotation is in a counter clockwise direction). Half the magnet will be seen outside the edge of the sensor and half underneath the sensor.
If you get a spark as the magnet is just starting to go underneath the sensor (right side of sensor) then there may be a defect with the sensor.

Okay. Here is what I do. I always check the timing of my engines before I install them on a plane. With the muffler off you can observe the piston by looking inside the exhaust port as it travels up and down as you slowly turn the engine over by hand. It is easy to see when it gets to the top before starting back down. There is also a slight play right at TDC for a few degrees as the piston stops going up and transitions to going back down. I believe Milton refers to this as TDC rock. If the muffler is on you can get TDC by removing the spark plug and insert something like a popcicle stick or toothpick so that it rides the top of the piston. You simply slowly turn the engine and watch the stick. When it starts to transition from up to down you are a TDC (verified by the TDC rock).

Okay, now you have the engine at TDC. I now put on the degree wheel and a pointer as have been described numerous times elsewhere. I set the pointer to zero degrees on the degree wheel. I have my degree wheel clearly marked at 28 degrees both left and right of the zero mark for easy reference. If you are at top dead center and the degree wheel is set at zero all you have to do is turn the engine clockwise to the 28 degree mark on your degree wheel. That puts the piston at 28 degrees BTDC which is where you want the spark to fire. Now simply look at the relationship between the magnet and the sensor. The magnet should be about half way outside the left edge of the sensor. If it is you are pretty much right at the 28 degree firing point.

Perhaps this is just a somewhat different way to check your timing and see that the spark is occurring at the right moment and may be a bit clearer to understand than some of the other ways that have been presented. I hope this helps.