If you read the article on duct sealing, you already know how extreme attic temperatures can get. As a reminder:

On a hot summer day, the attic temperature can reach 140°F (60°C) when the outside temperature is some 95°F (35°C) primarily due to solar radiant heating.

Building officials have come up with minimum guidelines to attic ventilation. The common reference is for every 150 sq. ft. (14 sq. m) of attic floor space (usually the same as the living floor space), 1 sq. ft. (0.10 sq. m) of ventilation is needed. Special circumstances allow for less ventilation, but this is a safe number. Ventilation includes: roof vents, turbines, soffit vents, ridge vents, and eave vents. In this example, continuous ridge vents are applied on the hips and ridges of a typical roof. A 1.5 in (38mm) slot is cut at the edges of the roof, then covered over with vents. The type of vents I used form a decorative architectural roofline, similar to thick hip and ridge shingles.