Please Please Please... The best and the easiest solution in case of #NA or any other error for the matter is to use "=IFERROR".
E.g. If you trying to divide 3 and zero (=A1/A2), you will get an error saying #Div/0!. All you need to do is insert "iferror" into the formula. Like this =IFERROR(A1/A2,"0"), Syntax=IFERROR(Value,Value_if_error). Thus Excel will return the Value "0" only if there is an error.
You can use any Value other than "0". "-" is a very comman practice. You can even have text, just add "There is a zero in division" and Excel will say so instead of "0".
You can use this for any Error, "#Name, #Ref, etc, and just about any error in excel.