Making one pose override another for certain bones?

I'm still wrapping my head around all the different things that can be done in Mecanim and I'm not sure which is the most applicable to what I'm trying to do.

Say I have a hand. And I have four poses: open, fist, finger pointing, and thumbs up.

Right now, I'm trying a direct blend tree which applies Fist, Pointing, ThumbsUp animation clips according to float parameters. For ThumbsUp and Pointing I define Masks on the import so they only apply to the thumb and index finger bones.

What's a good way to have it so if the ThumbsUp and Pointing parameters are 0, then the animation of the thumb and index finger blends between Open and Fist, but if ThumbsUp is 1, then the thumb is all the way up? Right now, the thumb sort of half-way goes down when you close the fist instead of doing a proper full thumbs up.

Can I not do this in a single blend tree, so I'll need to make a second layer for the thumb or something like that?

 

So then what would I be adding them to? The Open pose? Or do Open and Closed blend tree on one layer and then add the Thumb and Index fingers on the additive layer? Or ... ?

 

Okay, I did make two layers, one for open-closed and one for thumb-finger. But I think override looks more like what I'm trying to achieve than additive.