I hope I can explain this one...

In Column M (5 through 16)- I have 12 numbers which are being compared to
their counterparts in Column S (5 through 16). However these are monthly
figures so the months that are not completed yet are blank.

I need an equation to go into C21 that will compare the total entries as
they are filled in - for example until there are 3 entries in Column S, it
will only compare the Sums of the first two cells (5 & 6) in column S to
Column M to calculate the percent difference.

Is this possible?