Hi, I think you are mixing up the U coordinate, also known as the "time" of the cubic equation (or whatever you choose to go with) used under the hood, with a linearly distributed points on a curve (basically a post process done after the initial interpolation, length based instead of "time" based).
So, if you want to get "percentages" based on the length you will have to calculate it yourself on top of the existing interpolation given by softimage curves, You basically take the full length, divide it in discrete segments generating samples (corresponding to a known percentage) and remap U values linear interpolating to the closest sample point, the more samples the more precise the results will be (this is common practice, even softimage uses internally an approximated approach to get the length of curves by adding discrete segments). I think there's a linearly interpolated bezier compound in the examples that can be used as inspiration of how to go about it (or was in rray.de?). Although depending on the use case a custom node might provide a more performant solution. Cheers!
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