Can you share what the actual / proper calculation would be (like with a spreadsheet)? Itâ€™d be easier to see the math laid out before figuring out the expression.

Iâ€™m not personally familiar with loan calculations, but I know there is a formula type:
â€śCalculate Formulaâ€ť â†’ â€śLoan Paymentâ€ť. Here I pulled it up on a text element:

and then I chose â€śLoan Paymentâ€ť

and from there choose which fields the calculation is pulling from: