I'm using a SumProduct formula to determine a differential rate for a
threshold amount, but my table includes negative numbers, which throws off
the formula. Here is the table and formula I am using:

Column I/J/K
Threshold/Contribution/Diff Rate
<-3409 -750 -750
-3409 22% 22%
12400 25% 3%
15600 29% 4%
18700 34% 5%
21900 40% 6%
25000 47% 7%

=SUMPRODUCT(--(H14$I$4:$I$9), (H14-$I$4:$I$9), $K$4:$K$9)

Any suggestions appreciated.