Simple question from a novice

Hello, 

If someone can help me figure out an elemental problem here I'd be very grateful. The problem is as follows:

As a given, I have an array of planes that are in varying orientation. Because of the way they were created initially, they are not uniform in width. My solution is to find the center line, offset both sides the correct with, then re-loft. I've made a (perhaps cumbersome) expression that gets me up to the loft part; however I cannot get curves at the end to make the loft. Is there anyone who can push me over the edge to get this simple expression to work.

Thanks!