Source is Mayo Clinic they recommend if carotid artery stenosis has never caused symptoms, the artery should be repaired if its diameter narrows by 50 percent or more.
If the artery is narrowed by more than 50 percent but less than 70 percent, repairing the artery may help prevent future problems.
Artery Narrowed Less Than 50 Percent
There is no proven benefit from intervention to repair an artery that is narrowed less than 50 percent. Medical treatment such as taking one adult aspirin or other antiplatelet agent a day and controlling risk factors is often as effective as repair
Hope this helps.