Existing Center Line is critical information. We must be able to create such a line in an easy way without having to apply workaround, 3/4 steps workflow or MDL.
It means H+V+existing cant
In the past we had a single box to check to create this existing CL in addition to regression line.

The easiest would be a "Create Existing CL from regression line", assuming user always wants the regression line. We know everything from the regression line XYZ+existing cant if created from rails.

However, it could be useful to have same set of commands similar to create regression line but to create existing center line. We already have create rail line from points.
Why ? Because, in some cases we may already have the regression line that is not strictly the existing CL: points not used, starting on a track and transitioning to another, or any other case I don't have in mind.

So starting from the source, independantly from the regression line would be good.