Placement of ridge vents

Is there a rule of thumb when it comes to the placement of ridge vents on gable extensions? In other words, if the gable extension’s ridge is lower than the ridge on the main roof should you still install ridge vent. Lets assume that there is continuous soffit vents along the entire perimeter of the house. Does it matter how much lower or higher the ridge is from the main ridge? Also, sometimes the extensions are additions that have their own attic space which is not continuous (or just a small access hole cut in)with the main attic. If this was the case, I would assume that you maight treat it as a seperate roof when calculating your intake/exhaust vents. Any thoughts or suggestions would be appreciated.

Ok from my understanding of the rule this is what i know. If the roof’s attic is open to the lower area that ridge vent is going on to then you should not use ridge vent lower than 4 feet than the main ridge. If the area is blocked off then treat it as a seperate area and vent as you wish. This information i found years ago from Air Vent the makers of Shingle vent. Hope this answer helps.