Values Defined by Criteria dependent on salary

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We have a salary deduction currently set up as a Value Defined by Criteria. It's made up of a % of salary, plus a flat rate fee, with a maximum cap applied. We have a business requirement to change the deduction amount so that it varies depending on salary: there is one flat rate fee and maximum limit for employees earning up to £24,000 per annum, and a different fee and maximum limit for earning above £24,000. I thought I could achieve this by simply updating the VDBC criteria and having two 'from' and 'to' values with the rates. However,