How to Remap Two Shape Keys to a Range of 0 to 1
You can duplicate your object and make sure each of your two objects has the shape key value you want. The first is to be your 0
position, and the duplicate is to be your 1
position.
The following steps include destructive edits. Saving your .blend file before proceeding is advised.
For each object, from the Shape Keys menu's V-shaped drop-down button, select New Shape From Mix. This will result in a new shape key being generated. Delete all other shape keys for the object except this newly created one. Then lastly, delete this one too, so that there are no more shape keys.
Select your duplicated object, then shift-select your original object.
From that same drop-down button, choose Join as Shapes
Note that in order for Join as Shapes to work, the two objects must have matching vertex indices. This means you cannot add or remove vertices before joining. Only the vertex positions should be altered.
Your original object will now have both a Basis shape key and a shape key named after the duplicate object. (You can double-click a shape key's name to rename it if you wish.) Dragging the Value slider for the duplicate object to 1 will bring it to the other shape you want.
In this way, you will end up with a range of deformation that encompasses a "negative" shape, a "positive" shape, and a neutral shape somewhere in between.
The animation above illustrates a transition between a negative position and positive position. In this case, the neutral position is precisely in the middle at 0.5 on the Value slider.
For more about Shape Keys, see the Blender Manual.