Japan: the hensai futan ritsu
Japanese lenders work from the hensai futan ritsu, the repayment burden ratio: the share of gross annual income going to all loan repayments, not just the mortgage. Most set the ceiling between 30% and 35%, and lower it for smaller incomes.
The closest thing to a published rule comes from the Japan Housing Finance Agency's Flat 35 programme, which states its criteria openly: total repayments of at most 30% of annual income below 4 million yen, and 35% at or above it. Private banks are not bound by those figures, but they are the reference point the market works from, which is why the limit here is labelled a market convention rather than a statutory one.
The quirk that catches borrowers out is the rate used in the assessment. Japanese mortgage rates are extraordinarily low, but many lenders do not assess you at the rate you will pay: they apply a notional rate well above it, so the payment inside the ratio is larger than the payment on your statement. A loan that looks comfortable at the contract rate can fail the burden ratio at the assessment rate.
Income is assessed before tax, and the ratio is measured annually rather than monthly, the same arithmetic, but Japanese lenders and this calculator's annual setting speak the same language.