JohnTopley: The formula you provided does work perfectly, but I need help to make a modification. Turns out that numbers less than zero give an incorrect result in column "D". Can the formula be modified to ignore values <0, or treat them as 0?

For cell D14 in the sample data, the formula:

=IF($C3="","",SUM(OFFSET($A$2,MATCH(LOOKUP(10^10,$C$2:$C2),$C$2:$C2,0),0,ROWS($1:1)+2-MATCH(LOOKUP(10^10,$C$2:$C2),$C$2:$C2,0)-1)))

should return 415.

The negative value in cell A12 changes 415 to -1994.

Thanks for any help you can offer!