Could you post your grasshopper definition with the data internalized?
Have you tried the point on curve component?
From what i understand of your question, this might be what you are looking for.
What point on curve does; It takes some curves as input and then reparametrizes the lenght to a domain of 0 to 1. Then through the slider you can find point at any given percentage of the curve.
Example, a curve with a length of 500 units is being input to the point on curve (hereafter POC), if the POC slider is set to 0.1, it will find and output the point at 10% length, meaning 50 units from start-endpoint along the curve path.