This mentions the 30%, they should have mentioned it in the article, it's the standard amount they use every year I think: http://nlihc.org/oor/2014

Also, many places won't let you rent without meeting the 30% of income amount.

In some dense cities, the bulk of the transportation costs are rolled into your housing costs, because you are buying walking distance to a subway.

Yes, it really seems like a variable approach would work better. There's a big difference between spending 30% of your income on housing while living in a rural area and doing so in the middle of Manhattan. I imagine that percentage could go considerably higher in big cities.

It doesn't say that in the article...

It is an important point that would provide context to a lot of numbers.

Click "wrote about a report", then in that article click "new report", and you'll find this:

"The signature finding of Out of Reach is the annual Housing Wage - the hourly wage a full-time worker must earn to afford a decent two-bedroom rental home at HUD-estimated Fair Market Rent (FMR) while spending no more than 30% of income on housing costs."

It is not at all obvious. I really wish that news articles like this would set aside a section where they explicitly name and link to their sources rather than burying them as hyperlinks in the text.