Maybe try using something like:
SUM( Janunits + Febunits + Marunits + Aprunits + Mayunits + Jununits + Julunits + Augunits + Sepunits + Octunits + Novunits + Decunits ) + 0.000000001 AS CurrentTotal
and
Round((Difference / (CurrentTotal - 0.000000001))*100,2) = PercentageDifference
By adding an insignificant number to the result you can prevent ever dividing by zero without throwing off your calculation by more than would be significant after dividing.