The thing with timing lights and setting the engine timing is there will be a variation. A belt stretched a touch, timing marks not being 100% accurate, the head milled and refaced, a worn key way in the crank... you're sort of in the dark as to what it's actually up to. I use a timing light as a diagnostic tool now. It'll tell you if the engine is doing something funky but that's about as good as it gets. It does sound like you've got it dialled in as well as can be expected. The good thing about the 4G64 is there isn't any vacuum or mechanical advance that can screw up.