Adding the next x number of cells in a row together

Solution 1:

If you want cell A1 to evaluate the sum of B1 through U1 (the next 20 cells in row 1), you could put

=SUM($B1:$U1)

into cell A1.  But this probably doesn’t do what you want when you delete column(s).

=SUM(OFFSET(A1,0,1,1,20))

might be closer to what you want.  If you want the 20 to be a parameter, enter 20 into cell Z1 (for example) and put

=SUM(OFFSET(A1,0,1,1,$Z$1))

into A1.

Of course any of the above can be replicated into subsequent rows.

Solution 2:

if you have data organized by date, try a pivot table to massage it. You can more easily filter in the pivot field and columns, and you get to keep your historical data as a bonus. As you describe multiple values, the data would have to be transposed into one observation per line (as in forecast-monthly-v2.xlsx).

quick example: http://www.well.com/~fl/forecast/