Say NO to default..

I heard Dave Ramsey on Fox News yesterday say,
Your mortgage/house payment should not be more than:
1/4 of your bring home income, with your payment amortized on a 15 year note.

For example: using 5% interest rate
$100,000 Loan - 15 year note $ 791 principal and interest only (add taxes/insurance)
$200,000 Loan - 15 year note $1582 principal and interest only (add taxes/insurance)
$300,000 Loan - 15 year note $2372 principal and interest only (add taxes/insurance)

This sure would be wonderful in a perfect world. Right?