Required income for tenant - gross or net?!?

[David Sisson]

So, right now, I'm requiring that the tenant have 3x the rental as their income. I've also heard it said that you should require 35 times or 40 times the monthly rent as their yearly income (which is a bit more strict). But should this be NET or GROSS income? I've been assuming that 3x monthly rent should be the minimum gross income, but maybe I'm not being strict enough? What do you use?