Ok.

I just spent about a half an hour typing a bunch of things. And in the midst of all that decided a different approach. Here it is.

You should have created your fit morph around the natural scale limit value of Head Size, which is 1. Not 2.

When you create a morph and it is returned to you in Poser as morph target, what's it's natural parameter value matching the morph you created?

1

You created your fit around value 2. Now it won't fit value 1.

Recreate the morph around Head Scale value 1, it very much more than likely will fit at value 2 when it is dialed to 2 also. That's the way this works. If it isn't perfect at value 2, highly unlikely, it's infinitely easier to fix it with a difference morph added to the dependency. Any other unnecessarily complex hoop jumping trying to backtrack it to fit at value 1 from 2 will more than likely result in morph telescoping.

Do it the right way to start, this is a potential product after all.

Let me know how it goes, if there's still a problem the fix is VERY easy.