as a little disclaimer, this is my private take on this topic.
Cinema does calculate the length of a spline (at least with
SplineHelp) in a rather unpretentious fashion as the sum of the Euclidean norms of the underlying
LineObject segments, i.e., there is no fancy arc-length calculation going on. Which makes sense in Cinema, since the discrete
LineObject is what does effectively count, and not the smooth
SplineObject. Since you can get access to the
LineObject of a
SplineObject and also convert between
LineObject vertex indices with
SplineHelp.SplineToLineIndex() , this also means that you can both calculate the length of spline as a total and up to a specific vertex. Which then should mean that you can calculate the relative offset for a spline vertex yourself.
Practically you can cut out the
SplineObject altogether when sampling a spline and just use its associated
LineObject. The one nice thing that
SplineHelp does for you is though, that it implements a parallel transport for the frames of spline points for you. I.e., realizes that the normals of spline points point into a direction humans would consider "correct" (mathematically these normals are not correct, due to them being modified by parallel transport, but they will look smoother, won't flip when then curvature of the spline flips).