Start at around 10 degrees BTDC and work your way forward until you ping and back off a little. I would guess you will end up with around 15 degrees BTDC. Note, details like compression ratio, condition, grade of gas, elevation (yes, feet above sea level) plus many others are all factors that affect optimal performance. But between 10 and 15 is usually a good place. This is one of the advantages of the newer EFI systems, it does some self-tuning.