Okay if I am understanding you. You want to create a tube along a path and at each section (where there is a hull which you can control by setting where the cv's are along the curve) you want to "precisely" control the diameter of the hull?
Unfortunately, all you can do in maya with hulls is scale them. Maya is not a CAD program it's a artsy fartsy watered down CAD program for drawing things by eye in 3D designed for artists.
Now, you can control the size of nurbs circles from their creation node and so you could attempt to place a series of circles along a path and then loft between them but you'll have to manually go back between the spans and adjust the flow of the tube.
Why does it need to be so precise that you cannot just scale the hulls by eye? Can you describe or show a screen shot of what you are trying to achieve?
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675
Last edited by ctbram; 16-12-2012 at 11:06 AM.