43%是Fannie－Mae的标准，算法也是Fannie－Mae的算法。

One house is simple, just debt/income, debt = monthly payment of mortagage. 

Two house has two ways

(1)   (debt 1 + debt 2) / (income 1 + income 2)  so even debt 1 = income 1 (rental house), debt 2 being the house to be purchased, even if debt 2/ income 2 < 43%, the final result would be more than 43%.

(2) (debt 2 + (debt 1 - income 1) / (income 1 + (debt 1 - income 1)),  this way, net income of rental house is accounted for in total debt and total income.