Primary Residence Affordability Metric/Ratio

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What metric or ratio does everyone use to determine how much primary house to buy (rules of thumb)? 

From reading Millionaire Next Door awhile back, the author recommended total mortgage debt to be no more than double your annual income (pre-tax).

I was close to this when purchasing our first home a couple of years ago. I find this metric more useful than comparing monthly payments to income because it makes you more aware of the total purchase price of the home.

What are everyone’s thoughts?