This article may help you a little here;
How much attic ventilation is required to provide proper temperature and moisture control? A number of studies sponsored by federal energy funds are under way to look at ventilation rates and methods.
However, several studies previously made can help with this decision.
The maximum ventilation rate is required to remove heat during the summer cooling months.
Attics can reach temperatures of 150 to 160 degrees F during a summer day, although outside air temperatures are only 95 to 97 degrees F.
The cooling load for a home air conditioner depends on the difference in temperature between the inside and outside air, and reduction of attic temperatures from 155 degrees to 105 degrees F will result in a significant reduction in cooling load. In a home with poor ceiling insulation, heat movement through ceilings may account for 30 percent or more of the total cooling cost.
With a well-insulated ceiling, this source of heat may account for only 12 to 15 percent of the total cooling cost.
Thus, high attic ventilation rates are most important for poorly insulated ceilings. A poorly insulated ceiling is one whose R rating is less than 14 or one with fewer than 4 inches of fiberglass, rockwool or cellulose insulation. :)