I'm not sure what you're wanting to angle for here, that doesn't seem like a specific formula. If you're trying to calculate a single metric it seems to be number of years to pay off the debt? At current trend, that is infinity years. I'm not seeing how it could get worse than that, although no plans to pay back the debt is just business as usual by now. And I suspect the information in your sentence is actually contained in a Debt/GDP ratio (eg, is interest + GDP growth going up faster than debt implies Debt/GDP ratio dropping).
> I wouldn’t compare my $400k debt with my monthly income.
Pretty sure that is actually a fair way to assess your creditworthiness. I can see the conversation in the bank going "Yo I want $400k", "What is your income?", "$X", "Yea"/"Nay" with some basic due diligence.