Mortgage lenders use two formulas to determine if you’re likely to meet your monthly payments. In regard to the amount of monthly payments, the rule of thumb is that you should pay no more than 30 percent of your gross income. If your gross income is $3,500 a month, the mortgage payment should not exceed $1,050. Also, your total debt ratio should be below 40 percent – that is, the mortgage payment plus all the rest of your debt (school loans, car, other debts) should be under $1,400.