Partially dependent on the track, class, and driver size. In your case I probably would not go below 10. Some tracks have high G banked corners at the end of the straight like Jacksonville that I have seen tires come off at 12# at 360# in a 4cycle.
Also depends on type of wheel. Rims that take alot of air to seat the tire can usually run lower pressures. We had one set of wheels that were especially easy to seat the tires and they would also unseat with low pressures while racing at Jacksonville.