I count:
[(take home pay) - (total expenses including interest)] / (take home pay)
Take home pay = income-income taxes-health insurance
I don't count income taxes since I can't do anything to change them, and I won't be paying them in retirement. It would artificially depress my savings rate dramatically. But I also count property taxes and sales taxes, which are things I will have to keep paying. The health insurance is questionable to not count, but it's what I do. One reason is that the cost of health insurance will be very different when my employer isn't paying a portion of it. When I plan for FIRE I add in the estimated cost of health insurance.
So all loan principle payments, including mortgage, are savings. Whatever account I put it in (checking, IRA, 401k, investment, loan) it's still me saving the money instead of spending it.