I'd enter the formula

=SUM(A1:A3) / SUM(A1:A10) * A12

in an empty cell in column A, say in A15.

Fill or copy to the right to the last column where you need this. Let's say to Z15.

The total is then given by

=SUM(A15:Z15)

Regards, Hans Vogelaar (http://www.eileenslounge.com)