How much ventilation do I need on a small roof?
Venting should be 1/300 of insulated ceiling area, distributed uniformly on opposite sides of the building. (1/150 if slope is less than 1 in 6)

So whether you go with your 1/150 number mentioned in your question, or 1/300: using any vent larger than 8" x 8" at each gable end you are already going well above and beyond either requirement, so like sherlock homes mentioned in his comment, don't overthink it.

If you use a 12" round or octagon louvered vent at each end you will be safely above 1/300, 1/150, and 1/100.


  "The recommended ventilation ratio to provide for vented attic assemblies when an air 
  barrier is present, is the 1:300 ratio (as specified by most building codes).  This is based 
  principally on good historical experience and simple psychrometric analysis 
  (Handegord &amp; Giroux, 1984)." - Building Science Digest 102 (2006-10-26) - Understanding Attic Ventilation