You are probably going to have to upload a sample workbook, so we can actually see the different cell references, follow the circular logic, and possibly identify where the error is coming from.

Until then, something to consider -- I've found it desirable whenever I set up an iterative calculation like that to have error traps somewhere in the loop to catch errors and other problems. In this case, you want an error trap associated with the given division formula that will catch the div by 0 error, feed a reasonable value to the next step in the calculation to allow the loop to continue.