I believe you can find answers to your question here: http://www.ncbi.nlm.nih.gov/pubmed/18561343 (Normal reference values of ratios of blood flow velocities in internal carotid artery to those in common carotid artery using Doppler sonography) or here: http://radiopaedia.org/articles/ultrasound-assessment-of-carotid-arterial-atherosclerotic-disease.
You should consider these values assessed in a consensus conference about carotid ultrasound: http://pubs.rsna.org/na101/home/literatum/publisher/rsna/journals/content/radiology/2003/radiology.2003.229.issue-2/radiol.2292030516/production/images/large/r03nv36t3x.jpeg