Hello Shawn, this is Dr. Love and I will be glad to assist you today.
It is not a simple direct calculation to determine the stenosis from the velocity. The thresholds that have been established as the point at which there is concern that there could be a stenosis of 50% or more is based on assessing variability at the various levels of stenosis. However, it is not a perfect threshold, and this has been much debated and discussed in the medical literature over the years
If you are interested in reading about the specific issues with this issue, you can read an article at http://stroke.ahajournals.org/content/43/3/916.full.pdf+htmlec. Even without reading the entire article, though, there is a graph on the third page that shows the amount of variability in peak velocity that can be seen at each level of stenosis.
So, in reviewing the clinical information, the current recommendations were developed that includes the thresholds of peak velocity in the internal carotid of <125 cm/sec and a ratio of internal to external carotid peak velocities of 2.0 to indicate that the stenosis is likely <50%.
I would note that a 0 velocity is a special case. In general, as the stenosis gets worse, the velocity increases, similar to when someone holds their thumb over the opening of a hose to spray the water. However, if the stenosis becomes 100%, that will result in no flow, or a 0 velocity.
I realize that someone that wants a greater level of specificity can be frustrated by the variability in ultrasound determinations, but at present, that degree of accuracy can only be attained by an angiogram, and we generally want to avoid widespread use of angiograms, because it is more invasive and has a higher complication rate (the overall rate of complications from angiograms is low, but if done on a large number of people can still generate significant numbers, while the complication rate from ultrasound is essentially zero).
If I can provide any clarification, please let me know.