Hi Michael. Regarding fitting, I was only referring to Rhino's lack of user defined tolerance within the command. I've never seen it actually produce a result which didn't match at the edges. But it's unlike NetworkSrf which gives you tolerance options and can make a big difference in resulting surface match and density... About deviation, just speaking of matching shapes to required positions and angles according to design specs. If you need to precisely fit the interior portions of the surface to existing geometry, then Blend is not the best tool...

jonah