To use a result of a formula in an export formula, you will need to create a Part Attribute on the respective Export Part and write your formula there. Doing so will create a "this_part" variable that can now be used in the export formula. For example, if I needed the panel width in the export formula I would create a part attribute call "Panel Width" and write a formula like (door_width - (stile_width*2)+inset_add). That will create a variable named "this_part.panel_width" that can be used in the export formula section of the part.
