You could start with 6, drive it, see how runs, then bump it up to 8, drive it, and so on. When you get to the point you have a little spark knock during a hard pull, back it down to the last setting and you are good to go. The gasoline you use plays a part in this also, you can bump the timing up more with Hi test than with regular.
I am running 14 initial advance, mostly because with my Comp 262H cam it ran terribly at the stock 6 BTDC. At 14 I am still able to start well, the common wisdom is as much initial as will not cause bogging when trying to start and behave well under driving conditions as Carlton suggests.
I run 14 initial, 22 centrifugal and another 14 vacuum (Crane adjustable advance can). I carefully tailored each of these numbers to get timing that does not cause detonation.