I am getting this problem as well....I have even opened a standard part and copied and pasted the correct equation from there to my new part....and it still doesn't work. Anyone have a solution?
This issue has been reported to Autodesk for C3D 2013. I had the same problem when I was trying to change my wall thickness I go the same error. So since I could only set one constant wall thickness, I just add the difference to my structure inner width and length (SIL/SIW). For example. Most of my structures walls are 6 inches but I have a 7' junction box with 8 inch walls. To fix this issue, besides using 84" for the inner dimension (SIW/SIL) for my box, I used 84.34 inches. Now my outer structure limits are correct. Maybe you can try this method.