Using a variable number of cells in SUM function

Cell D2:

Formula:

=IF($B2<>"","",SUM(INDEX($B:$B,ROW($A2)+1):INDEX($B:$B,ROW($A2)+$C2)))

and copy down.

Or

Cell D2:

Formula:

=IF(ISNUMBER($C2),SUM(INDEX($B:$B,ROW($A2)+1):INDEX($B:$B,ROW($A2)+$C2)),"")

Variations on a theme depending on your data structure.



See the updated example attached