Offset with pufferfish

I am using the offset surface function in Pufferfish to create a solid. I want the offset surface to be larger (always) before the solid is created, but sometimes the function extrudes the surface inside as shown below:

Versus the correct method below

When I change something in the base mesh (of which this geometry is derived), the offset distance randomly goes internally or externally, forcing me to manually negate the offset distance. This is obviously tedious and unreliable. So how do I make sure that the offset distance is always correct, in making the offset surface larger?

Offset will go in the direction of surface normals. Your changes are causing Rhino to change your surfaces normal directions, so be sure to be aware of that.

ah, a poor approach is to generate both surfaces (supply both positive and negative offset distances), then filter for largest (or smallest) volume