What percent of an applicants income to rent do you want?

[Jeff Gebhart]

Do you require that household income be a certain percentage of the rent in order to qualify them?  My Dad told me 25 years ago not to buy a house that the mortgage payment is more than 25% of your monthly take home.  Not sure where the science of that number came from but seems to be a good rule of thumb for someone to live within their means.  What is your rule of thumb on income when qualifying prospective tenants?