Tiered Rebates and Sliding Scale?

I'm trying to determine the best way to handle this request - any input would be appreciated.

The user is manually calculating rebates now, but would like to be able to automate some of the process. Their current rebate structure is based on total sales for all child accounts under a single parent.

The amount of the rebate is a sliding scale percentage - of the total sales of all of those child accounts.

100,000 - 200,000 2.5%

200,001 - 500,000 3.0%

500,001 - 999,999 3.255 etc.

I'm able to set up a tiered structure to accumulate at the parent level, but I'm struggling with how to handle the sliding scale part.