Hi Michael,
When you created your Debt Account, you entered a current Balance and an Interest Rate for the Account. Then, when you go to make a Debt Payment transaction, if you fill in the total amount you paid in the Amount field at the top, the software will automatically calculate what of that is being paid to Principal and what of that is being paid to Interest based on the Interest Rate you provided. So you shouldn’t have to manually enter anything in those fields unless you want to make a correction or change.
In other words, Goodbudget should help you track those Interest payments automatically, provided you have a constant Interest rate.
Does that help and make sense?