calculating a sliding scale percentage based on value of a sum

This can be done easily without hard-coding percentage breaks into nested IF statements by setting up a table defining the rates payable for various sales volumes.  Something like this:

Which I named Rebates.

Excel tables allow for structured referencing within formulas.  This means you can use the table and column names when typing formulas by typing "r" (without the quotes) and excel will suggest functions, ranges and table names beginning with R or r.  Continue typing the name or select from the dropdown to accept "Rebates".  Type a left square bracket "["and you will see a list of columns in the Rebates table which you can select from and close with a right square bracket "]".

INDEX/MATCH formulas are a bit like VLOOKUP but more flexible in that you can return values to the left of your MATCH and specify exact (0), less than (-1) or greater than (1).

The formula in column B below calculates the rebate percentage applicable for the sales volume and multiplies it be the sales volume.

Much easier to maintain than nested IFs.