richard,
your example is fine! i am thinking along the lines of what scott said. i also agree totally with your picture and the way it shows high and low venting. if the upper venting is not three feet above the soffit venting then 1/150 must be provided.providing the full amount of venting at the soffit or eaves only can have detrimental effects to the roof coverings. how many times have you seen curled edges on comp roofs towards the peak of the roof and discovered that there were only soffit vents?what was your opinion on why the shingles were fine until half way up the roof? did you attribute the product failure to high temperatures in the attic due to inadequate design of the roof ventilating system?