How to define 50% Rule

[Nathan Grubb]

I have a question about how we are defining the 50% rule. 

For most people I talk to in normal, non-forum life, "mortgage" refers to PITI.

When the 50% rule is defined as (gross - mortgage) x 0.50 = expenses, are you considering that variable of "mortgage" to be solely the principal, or the more colloquial "mortgage" of PITI?