I have a master's degree in Engineering, so formulas are a breeze. I helped my wife who is a professor with grades and formulas so this lines up perfectly.
I am setting this up as $15 when all the questions and source material are address due as a 50% deposit to start the work. I would want the data in csv format, spreadsheet, pdf, etc.
The rest is due when you are 100% satisfied with the work.
Since clients only pay when 100% satisfied, you may see my completion level at 50%. However, keep in mind that these clients ended up with 95% or more of what they asked for at only the nominal cost of the deposit, so they ended up with a great deal. More than my reputation, your satisfaction is what is most important to me, and I am looking forward to repeat business also!