Global variables and equations in assemblies - design tables

I have an assembly with a global variable: A=1.

I want to drive parts in my assembly with this variable.  ie. "D1@Sketch1@Part0.Part" = "D1@Length"-A  (Where 'length' is a distance mate, that is a column in my design table).  I have D1@Length, D1@Width, D1@Height as column headers in my design table.

I would also like to drive the value of A with the design table.

I created a column header in the design table called "\$VALUE@A@equations".

When I exit the design table I get a warning in the upper right of my screen that states, "Column heading "\$VALUE@A@equations" contains an invalid equation specifier.

Exiting table without completing model update."

I'm pretty green to using variables like this.  I'm not sure if I have the syntax incorrect....or I'm just totally off base. 

SolidworksAssemblies