The 3x Rent Rule

[Arnie Guida]

I've wanted to ask this for a while but have been reluctant for fear the answer should be obvious, and/or it's been covered ad nauseum...so please accept my apology if either are true.

With the 3x rule, how is it affected by the number of people living off of that income? A single person by themselves making 3x the rent is much different than a young couple with two toddlers making 3x the rent...one mouth versus four mouths on the same income.

How is the income to rent ratio figured "per mouth to feed"?

Thank you,

Arnie