I guess there's a lot of complexity I'm missing. Is it something like this:

1. Customer original bill (variable - user provided) e.g. 100

2. Saving (variable - user provided) e.g. 9

3. Saving percentage (calculated) e.g. 9%

4. Billed Percentage (variable based on savings percentage?) e.g. 25% x 9

So you would have a billed percentage "rate card" that depends on variable factors like the customer bill plan, the amount/percentage saved etc? Is that the tricky bit?