Extrude scale varience
 

I'm trying to make an extrude have varying widths without having to use lofting. Basically I've made these veins, and I created a cirle and a path and am extruding the circle along he path. Then I've set the extrude scale to 0.15 so that it tapers along the extrusion. However, I'd love to have the width of each vein vary along it's length. Now it still has to taper at the end, so I'll need some control over this thing.

I figure the expression would be based on the "Curve from Surface" value that you can set to define where along the path your extrude starts and stops, however as of yet I have had no success whatsoever.

Any thoughts on how to write an expression that?

Thanks for the help,
-dann