I thought you wanted to control the location of an inner part entirely enclosed within an outer part. In that case, isn’t there just one dimension from the floor of the outer part to the floor of the inner part?  The minimum value must be greater than 0, or else the two floors touch, and the maximum value must be less than the thickness of the outer minus the thickness of the inner part or else the inner part breaks out.

I recommend you do this in DesignModeler and not SpaceClaim because if you make a mistake and enter a large value for the dimension that puts the inner part outside the outer part, you can’t type in a small value and recover the geometry if you use SpaceClaim, it becomes broken.  If you type in a large value in DesignModeler, the geometry recovers after you type in the small value.

In the DesignModeler history tree, the last step is a Boolean subtraction to make a cavity in the outer part using the inner part.