Depends on the efficiency of the over all package and particularly the head is as to how much timing it will want. The more efficient the less over all timing it will need to compete the burn. The less efficient it is the more timing it will need. There is no Magic number really way to many variables. But start safe and advance as needed. Many -1 head combos seem to end up in the mid 30's total fwiw some will need more some will want less. Remember timing will affect AFR's as well, the more it is advanced the leaner things will become generally speaking.