One other tip - if you plan on thickening this shape up later, it may be better results doing that if your basic surface does not pinch together at the top and bottom - shelling and offseting surfaces tends to be sensitive to tips like this that collapse down to a single point.

If you want to make shelling work sometimes right now you will have to create a more rectangular base surface that is not pinched at the ends, and then trim it to make your curved outline.

That kind of non-pinched-at-the-tip underlying surface with a trimmed outline will tend to shell a lot more reliably.

- Michael