sum of actual months with reducing numbers of forecast months

Hi

Can someone provide a formula that can produce the result as shown in cell P4? what i need is
for eg,,
when jan 2010 is the only actual month, the formula will be =Dec -((jan+((Jan)*11months)) where Dec is the column N which shown YTD

Feb 2010, the formula will be = Dec - ((Jan+Feb)+((Jan+Feb)*10months))) where Dec is column N
and so on.

Cell P4 is a simple formula which need to be changed every month when the number of actual months change. Can someone provide a more sophisticated formula that will change automatically when actual month changes?

Thanks

I don't see what you're trying to do. Also, you have Dec forecast twice (?), and the result of your formula in P4 is negative (?).

Maybe =SUMIF(B3:M3, "Actual", B4:N4) * 12 / COUNTIF(B3:M3, "Actual")