The best setting I have found so far is 2, with a distance cutoff of .003. I've only been using Poser for about a year, so I'm still a relative newbie, and I still haven't read the manual either. I don't know what size a unit of Poser space is, I'll look into it. I was just thinking that if I put it on the wireframe view, I could see how far apart the vertices actually are, and make a better guess.
After matching the face with raycasting and the head without (because of the ears), on something close to the default settings, and then merging the files, when I apply a morph that affects the size or shape of the head, Apollo gets either a bulge or a dent on his head. This is after running it through restore detail.
What have I done wrong? It's like the vertices where the face and head meet have been left out.
Does what you said about raycasting not working well for ears still apply if it is the second run and I have gotten them perfectly matched by copying the head shapes of both figures and making them meet halfway? I'm thinking I need to do the head and face together, unless there's a remedy for that problem.