Here's an example tree:
http://s3.darkvertex.com/hlinked/ice/ICE_example_kinematics_pathOrCurveUConstraint.png
You would have to use the UV To Location node to turn a U value (0.0
to 1.0) to a location then to a position and so on. Divide your
desired percentage by 100 and you get the U value.
Hope that helps.
Cheers,
-- Alan