Attic Ventilation rule...
Area of ceiling adjacent to the attic divided by 300 for a home less than 20
(divide by 150 for older homes)
Take resulting square ft and divide by 2
This will give you the required square feet of upper and matched reqired
ventilation are for lower ventilation.
Example: 2000 sq ft of ceiling / 300 = 6.7
6.7 / 2 = 3.3
The example home needs 3.3 square ft (480 sq inches) of upper ventilation
and the same for lower ventilation.