500 Serp ignition timing?

Hi, I thought I heard some pinking at Millbrook this weekend and so have checked the timing with a strobe. it was set at about 9 degree BTDC. The bible says 4 degrees on unleaded, so I have set it at that and all seems well.
But as a check I did a search and there is lots of reference 8 to 12 degrees BTDC.
What should the setting be for a standard 500 Serp?

Thanks for your help!

I will set it to 8 degrees, that is at least 1 degree less than it was originally, maybe nearer 2., and check the vacuum advance for operation.
Thanks for your help it is much appreciated!