3X the rent versus 40 times the rent?

[Julie Verardi]

I had always done the 3X the rent rule for what I can afford for a mortgage and for my tenants. For example  $2000 x 3 = $6000 (monthly income required to keep housing payments less than 1/3 of income)  $6000 x 12 months = $72,000 (annual income required to keep housing payments 

I just recently read this article 

http://www.businessinsider.com/7-simple-steps-to-d...

And they are talking about 40 times the rent. I'm also not the best at math but going by different rules you can afford a lot more with 3X the rent versus 40 times the rent. What do you guys think is the best way to determine what someone/ even yourself can afford?