I'm trying to come up with a formula that will calculate a total from a table
so if you had a figure of 9600 and you compare it against the table
4000 0%
2000 10%
5000 20%
5000 30%
10000 40%
it would return the answer 720 (First 4000*0%)+(Next 2000*10%)+(Remaining
3600*20%)

I found an excellent piece on Progressive Pricing on Chip Pearson's site but
despite a lot of tinkering I haven't been able to make it work for what I'm
trying to do above.

I'd be grateful for any ideas on how to do this.

Thanks a lot