Yeah, this is a perpetual dilemma. The shop manuals give an initial timing spec, with no centrifugal advance. Then, if you check the advance curves for the distributor, the normal idle speed gives a couple degrees of centrifugal advance! I suppose you could slow the sucka down to 600 RPM or so, but that's not really the point.
You have to understand that, even apart from that problem, the fuels we use are not the same as the ones in the 1960s, and this will affect the optimum timing. Also, in consideration of the fact that many engines are modified, the timing spec is not holy writ. Best thing to do, as Bob suggests, is advance it as much as possible without pinging. That cool little adjuster on the distributor makes it easy. That's one thing Lucas did right!