I am using linear interpolation between spline keys. We know all about location, but we know only 2 distances. So I assume that the part of the spline between 2 spline keys is quite linear.
Just draw a sphere on a calculated distance along a spline and compare results.
I did not get the point about fraction - I really have no idea how to use it and when I was trying to use the solution above - calculated distance had huge inaccuracy and was jumping time to time between some reasonable location and the previous spline key distance.