Total monthly debt payments and gross monthly income—DTI % (simplified front-end style).
Yes—debt must be ≥ 0 and income > 0. Ratio = (debt ÷ income) × 100 as a simple percentage. Which debts or gross vs net income to use is up to you; the script does not classify mortgage types.
This calculator uses plain decimal numbers. If a result looks wrong, check whether you used a comma instead of a dot (or vice versa) where the field expects one.