The conservative rule of thumb is %25.

I've stayed closer to %20 and it's allowed me to support a family of four for 20 years on one income while riding through two employment layoffs during that time.

Also, rather than spending our proceeds from the sale of our home in 2005 I put it back into our new home once the market was near the bottom. The net of all of it was that I was basically able to cut about 18 years off our original 30 year fixed rate mortgage.

That is the usual number given as "responsible". And these days? A reputable bank would not even consider approaching 50, let alone 60, per cent.

The metric is DTI (debt to income), and the magic number is 36%. However, this is pretax income, not take home pay.

Can you tell me why DTI should be <= 36%? Is it a fixed value of some regulation or is there a formula behind it?

It’s a standard used by mortgage underwriters, presumably based on analysis of decades of past mortgages they’ve written and calibrated to balance their risks and rewards for maximum profit.