It is generally not a good idea to override the "thickness" variable; it would be better to modify this in the sheet metal tool you created it with, such as "Sheet-Metal1", or "Base-Flange1". Sheet metal is unique in that it is the only time a variable (thickness) is automatically created and can be linked to other features. After you create a sheet metal part, sketch a shape, and pick the Cut Extrude tool, and you will have a check box for "Link to thickness" for example.
If you use a sheet metal gauge table, then it would be good advice not to have the "thickness" variable in a design table because the design table will override the sheet metal gauge table (which also has your bend radii). Our sheet metal gauge table matches our tooling, so that is why I don't recommend it.
Flores
SW06 SP3.0