I am a student and I found on this forum how to calculate the sum of the lengths of the polished lines using the "Data extraction" command. This worked very well for polished lines, by also selecting "circumference" in addition to "length" and "layers" I also managed to make the sum of the arcs of the circle, but it didn't take into account the "splines". Do you know a way to fit them into the total?
Have you tried the "length" function? (or _length if you're on a French version), I'm sure this function measures the length of a spline (I've already done it), but I don't know if you can select multiple curves