The repayment cover calcutation is usually 1.25x times cover (ie rental / repay). However as most rentals dont cover repays and most investment properties are now interest only, its usually now accepted at 1.25x cover on interest basis. In terms of support from personal income (if required) theres no hard and fast rule and banks are now working from available income up (rather than the previous salary multiple rule) the rule of thumb is 1/3 your income on repays only.