My father used to irritate me. I always liked to set ignition timing by the book, he'd set it by trial & error. He'd set it, take it for a spin, advance it, see how it felt, was it pinking etc., repeat a few times and settle on whatever he felt ran best.
His argument was that although the manufacturers settings were best when new, by the time he bought whatever it was it was old and worn. tolerances had changed, compression might be lower, adv/retard springs might be weaker and petrol might burn faster/slower/hotter/cooler. I don't think I'll ever have his feel for mechanics but I still try setting it a degree or two either side of correct just to see where it runs best.
So here's two questions.
1. what matters with timing - the travel of the piston or the time it takes to do it?
2. does modern petrol burn faster or slower?