As a test I've done to see how it affects things I've tried all manner of positions from really retarded to as far advanced as I could rotate the pump. You'll know when you've gone to far either way. Too much retard will make it hard to start and cause white smoke when engine is warming up or even worse at all times due to unburnt fuel. Too much advance will sound like you have a bucket of bolts sitting on your engine and blows a lot of black smoke/soot. From what I've found, retard in general normally gets you higher EGT, engine noise is quieter, faster to build boost and less smoke. Advance generally gets lower EGT, more rattle from engine, slower boost response, more smoke. It was actually a really interesting experiment and you can really feel how it affects the way the car drives.
It's much easier to see and adjust this on a dyno but I have found it pretty easy to tell the difference between each adjustment while driving using the same stretch of road each time and also timing your runs for certain speeds. The setting that gave me the performance I was after is a little rattly on very light load but clears up all other times. With my pump set around the listed specs in the FSM the performance is greatly reduced compared to where I'm at now, but I also have an aftermarket rebuilt pump and lots of other changes so it's not really any surprise the original static timing specs don't give me the best performance for my setup.
I'm not qualified in anyway to offer professional advice on this subject so do this at your own risk. I like to tinker and learn about this stuff by working on my own car and fully accept any consequences that come in doing so but if you're adventurous and like tinkering have a play with different levels of timing and see how you go. If you don't have a dial indicator to measure where you are now just scribe a mark and use that as a reference.
