This might help if you want to preserve the trajpar value of 0 to 1.
The cyan curve and related formula will create a "half height" cosine wave offset to invert it.
Play with the squared function by using cubed or higher power and you will get a variation between the start and stop.
The examples below are just the datum curve equations, but you would substitute the "y" entry for any "dn" dimension and replace "t" with "trajpar" in a VSS. Using the cyan equation in VSS relations will assure a tangent/normal start and finish but otherwise equivalent to the normal 0-1 range of trajpar.
