Advanced Tiered Pricing

I've been banging my head on the wall with the best way to accomplish a tiered pricing model in Excel, but I need a slight variation.

I can use the SUMPRODUCT function to create the calculation, as explained here: http://mcgimpsey.com/excel/variablerate.html

1-20 units = 12$
21-40 units = 10$
41-60 units = 7.50$

Value in A1 = 32

1-20 = 20 units @ 12$
21-40 = 12 units @10$
41-60 = 0 units @ 7.50

This method works great! ... HOWEVER... I want to be able to put in another value besides the total units that represent how many units in to BEGIN at. (B1) Example:

Value in A1 = 32
Value in B1 = 10

1-20 = 10 units @ 12$
21-40 = 20 units @ 10$
41-60 = 2 units @ 7.50$

My end goal is to create a sheet for helping customers with pricing, when they already have credit for previous purchases.

Any ideas?
Thanks!