Most spine reads assume the lowest disc is L5 to S1 and move on. Usually that assumption is right. Sometimes it is not, and when it is wrong, the surgeon operates at the wrong level and the pain physician chases the wrong nerve.
Anatomy varies. Transitional segments, an extra rib, a sixth lumbar type vertebra. These are common enough that guesswork is not good enough.
There is a better way. Count down from fixed landmarks that are correct nearly every time, and confirm the level using nerve root morphology, a method the literature supports with high accuracy.
In our practice we do not approximate the level. We establish it, because a neurologist following a specific nerve needs to know exactly which one we are talking about.
Precision here is not showing off. It is the difference between treating the right problem and the wrong one.