Hi Justin, I guess this would involve a lot of different calculations, some on detecting the curvature of surfaces, and others trying to analyze the interference between different adjacent surfaces.

Unfortunately these calculations are really complex, it would be really hard to actually implement this, even harder than doing the shelling operation itself...

But I do want to figure out some way to improve this in the future though.

In the meantime, there is an alternate method to entering a numeric distance - you can pick 2 points in a viewport, and the distance between those points will be used. This can help adapt to the scale of your object for example by picking the 2 points snapped on to a piece of your object...

- Michael