Ask the assistant to do the arithmetic as work, not as prose: the inputs, the code, the actual results, and the checks performed. Writing code and running it are separate steps — “here is the script” does not mean the script was executed.
A report can accurately describe a recurring saving and still mislead the reader about the first year’s total. You do not have to know a programming language to ask for the work; you do have to ask for the spreadsheet you can inspect.