It can be as simple as "total planned spending - all other expenses = how much you can spend on rent". As an example, we plan to spend 40k a year. Subtracting all our other expenses, we could spend 24k a year on rent without a problem. That comes out to 60% of our planned spending. We haven't applied for any housing run by big companies so I can't say whether that percentage would create problems getting approved for a lease.