The first sentence translates to "for the purpose of calculating the debt and the maximum finance charge..." In the context of qualifying a borrower for a loan and creating a disclosure of all the possible costs (especially if it is a loan with a variable rate), the lender (in the U.S.) generally uses a generic rate (here the 4%), which may or may not be the rate that ends up being charged in any given month. Thus, the translation I've suggested. Hope that helps. Suerte!