function:Simple Macro

I need help with a simple Macro (or formula). I will be updating the 2nd
comlumn as the months go on, therefore I want to change the formula in column
4 as the data is filled in the 2nd column. Is this possbile? (First the
formula would add the 500,000 + the 400,000 and then next month I would
change the 400,000 to 300,000 and the formula would then add the 700,000 and
the 400,000.)

Months 2006 YTDActuals Input End of Year 2006
Jan - Jun 500,000 400,000 900,000 / 1,000,000
Jan - Jul 700,000
Jan - Aug 0
Jan - Sept 0
Jan - Oct 0
Jan - Nov 0
Jan - Dec 0