Tech info wanted; Lean Burn Dist

A word about this info. Even if your timing light is not accurate, as long as the inaccuracy is consistant, what difference does it make? I use the same timing light on everything/everytime. So if it is off by any amount, it is off the same amount always.
How do you know what timing the engine wants? Trial and error, right? Make a pass check the plugs for signs of detonation, try a different setting, make another pass, see if the run is better or worse. Repeat. Now you know what the engine wants, so shoot the timing mark. Who cares what the number is. it could read anything, makes no difference, as long as you use the same timing light the next time. If you disturb anything just reset to the number you had previously.
PS: I do have an "old" Sears timing light. Don't know the model number.