Calculate the B-20 qualifying rate, qualifying payment, and resulting GDS/TDS from a supplied principal, contract rate, amortization, and income. Use this tool only when the qualifying payment must be calculated from those inputs. If the user already supplies a monthly mortgage payment and asks for GDS/TDS, MUST use calculate_debt_service instead and must not request principal, rate, or amortization. This is neutral scenario math, not an approval or underwriting decision. Use only with explicit, non-identifying numeric inputs. Calculation only: never use this tool to approve, deny, underwrite, recommend, select a lender or product, or fill missing inputs from prior chats, files, accounts, documents, websites, or web search.