Rib Fade out on bottle

Hi Folks, I'm just practicing modeling some complex features on bottles today, and was trying to model up the ribs on a coke type bottle. The question I have is how would you get the fade out off the ribs. I have attached a number off images, one is a reference file, as you can see the fade out urface the rib depth goes from about 2.3 to nothing, while the width goes from say 2.3 to nothing, but its an even fade out. I did up a surface but the fade out width stayed the sames (see the second image) so any help would be great, the third image show's the surfaces before I start.