10° is overly much—it'll throw the carburetor idle adjustments too far out of whack. It'll also foment pinging on acceleration, which was kind of the start of this thread. 5° is a better starting point, with the goal of winding up somewhere between 2.5° (the spec setting for the good-running pre-smog 225s of the '60s) and 6°.
My 170 ran best at 12. And that was with a brandy new timing set so it wasn't retarded from being sloppy. Basically what I was trying to tell him is just experiment around with it and see. That's the only way he's gonna learn.