Plumbing Vent – Proper service

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 & Giroux, 1984)." - Building Science Digest 102 (2006-10-26) - Understanding Attic Ventilation

Searching online using the phrase ""Transite flue pipe" leads to a large number of hits that indicate this type of pipe is for sure an asbestos cement pipe. It does need to be handled as a hazardous material when working with it and removing it.

Transite piping was commonly used as a flue pipe and chimney liner for gas appliance venting. It has had to be discontinued from use because the inside of the pipe would degrade from age, use and temperature changes and then start to flake off. With enough flaking the pipe could become blocked and cause serious problems with gas appliance back drafting.

Check with local requirements with regard to disposal requirements for this material.