Attic ventilation in
A simple example would be a 1500 square foot ranch style home. This would have 1500 square feet of attic so 1500 square inches of vent would be recommended. 750 square inches inlet and 750 square inches outlet, 750/75=10 so you would want 10 8 X 16 soffit vents and 750/85=9 12 X 12 roof vents or approximately 31 feet of ridge vent
Just because you count your vents and think you have the appropriate amount of ventilation you may be wrong. Take a look at the actual ventilation that I had on my house:
Note the outline of the vent and the real size of the hole in the soffit. And remember to subtract 40% for the actual vent.
Here is the hole enlarged to it’s maximum size. The 2 X 4 you see is the frame of the soffit. It does not affect the ventilation appreciably due to the vent will not touch it.
And this is the new vent installed