How house affordability is calculated
Take the smaller housing budget allowed by your two editable income ratios. Subtract property costs, then convert the remaining principal-and-interest payment into a loan balance. Add the down payment; keep closing costs outside it.
Worked example
At $10,000 monthly income, a 30% housing cap, no other debt, $500 monthly carrying costs, 0% interest and ten years, $2,500 per month supports $300,000 borrowed. A $20,000 down payment gives a $320,000 budget.