Importance of Debt to Income and Proper way to Calculate?

[Jim Goebel]

What is the 'proper' way to calculate Debt to Income, and what is its importance as one moves along with a buy and hold portfolio?

I'm trying to be prepared with a framework that would give us an 'exit' strategy, or at least a more quantitative analysis that would lead us to sit on the sidelines (beyond traditional ROI) metrics, and drive me to do something else with my time.

Intuitively I feel that the debt to income and what happens to it over time (ie: does it plateau as we increase in scale) seems important, I just haven't been able to put my finger on how to think about this.

Please, any and all 

And more specifically, if we were to think about debt to income, do we calculate income AFTER accounting for mortgage (debt servicing) payments, or before?  In other words should we be looking at debt to income based on NET income, in which case we'd kind of be 'double counting' the debt servicing, it seems?

Perspective specifically by commercial lenders might be helpful.  But please anyone that has something useful to share....