Hi billis, I'm not sure if I completely understand, but I think that you are looking for a way to have more control over the width of the ring near the bottom part.

The method that I showed there using "Rail revolve" uses the fewest possible number of curves. That makes it also the easiest to set up, but with a smaller number of curves also comes a sacrifice in the amount of control.

There are other construction tools that you can use instead of Rail revolve that will let you specify a larger number of curves which will give you more control. However, these tools are also more complex and may take more practice and time from you to learn how to use them.

The 2 main tools that you would use in this situation to gain more control would be either Sweep or Network, I would focus on Sweep in this case.

To use Sweep you need to have 2 "rail" curves that act as a guide to slide the curves along. So in this case, I copied what used to be the revolve profile over to the other side to make the second rail:




The basic shape of the sweep will come from the ellipse at the top traveling along these 2 rail curves.

To gain more control I will use an additional profile that I draw in the right-side view, like this:



This additional side profile will be applied to the sweep using the "Scaling rail" option - this will cause the sweep to stretch out or shrink down so that it conforms to that additional profile. By adjusting this profile you can gain a lot of additional control over the shape.

You should now have a set of curves that look like this in the 3D view:



To perform the sweep, start by selecting the ellipse at the top - this is the profile curve for the sweep.

Next, run Construct / Sweep. The prompt in the upper-right corner of the screen will say "Select one or two rails" - select the 2 rail curves and push Done (or right-click inside the viewport which is a shortcut for Done). The basic sweep will now be displayed:



You can see that it is currently quite similar to the rail revolve. Now before you exit the sweep command, you want to apply the scaling rail to adjust the shape to conform to your additional profile. To do this, push the "Pick scaling rail" button circled in red above, and then click on the scaling rail. The sweep will now update with the additional profile applied:



You can now push Done (or right-click) to finish the sweep. You can now edit the control points of the scaling rail to adjust the shape some more, the sweep will update if you edit any of those curves that went into it.

For example here I have edited the scaling rail to make the shape quite narrow:



It is also possible to use more than one profile in Sweep as well, like you could place additional ellipses coming down from the top one and use all of them. But the scaling rail usually takes the place of drawing in a lot of profiles.

When you are done making the base shape, select it and use Construct / Boolean / Difference to slice the circle out from it just like the previous example, shown here with fillets also applied:




I attached the .3dm file with the curves in it here as sweep_ring.zip .

Also check out Jesse's tutorial here: http://moi3d.com/forum/index.php?webtag=MOI&msg=510.5 , which goes through a similar type of setup.

Hope this helps!

- Michael