Allocate Rebates Formula

The number of transactions is scaled down to provide this example.

Rebate scheme (Cum Sales = Cumulative sales) based on the total turnover of a group of customers.

Cum Sales....Rate....Sales Band...Rebate
20,000........1.0%.......20,000........200
35,000........2.0%.......15,000........300
45,000........2.5%.........3,400.........85
...................Total......38,400.......585 average rebate 1.5%

I would like to correctly allocate the £585 rebate across the individual customers who have generated the turnover i.e. NOT by using the simple 1.5%.

Clearly the large sales value customers will have earned a higher % rebate than the lower sales value customers - but how to calculate?

Customer....Sales value...Rebate
A...................15,000
B.......................900
C....................7,000
D....................3,000
E.......................500
F.....................9,000
G.....................2,000
H.....................1,000
Total...............38,400.........585