Attic venting question

I am trying to figure out what I need in order to improve the ventilation of the attic space in the house I have recently purchased. It is about 50 years old, and not surprisingly, it is not vented very well.
While researching this some I have come across the rule thumb that there ought to be 1 sq. ft. of ventilation for every 150 sq. ft. of attic space. What I am not certain of is if that rule of thumb includes both exhuast (such as ridge vents) and intake (such as soffit vents). It seems clear to me, also, that the ratio of exhaust to intake must make some difference.
Can anyone is this group give me a little more detail in the general guidelines for properly ventilating an attic.
TIA
James
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Look at the Building Science Corporation web site. This is * researcged * information.
Not every climate is good for venting attics. If you vent an attic, exclude infiltration from the house. Install a vapor barrier. Vent fans and plumbing to the outside.
TB
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Have you considered "COBRA" ridge venting?

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