I don't think there's an easy formula for a very simple reason: everyone's preferences and risk tolerance of running out of money is different.
You mention you have a 175k mortgage. This could be really bad, or really good depending on a few things. For example:
- How much of a nest egg you have saved up?
- What income streams you have and how big are they?
- What do you expect your investment returns / dividends to be like for the rest of your life?
- With all this, do you feel like you'll run out of money before you run out of life?
When you answer all these questions, be sure to add a safety margin for financial emergencies! This safety margin is up to you and your own personal preference / risk-tolerance, which is why a straightforward formula is pretty hard. But if you must use a formula, on a high level it's:
Your net worth - ($ spent/yr * how many years of your life is left) + ($ coming in per year from pensions/dividends * how many years of life you have left).
If the number is much larger than $0, you might not run out of money before you run out of life. If it's negative, the reverse is probably true. Though if you're not sure of the math, you should really talk to a financial advisor and make doubly sure that you have enough resources for retirement before doing so. You can only retire once, so make sure everything's lined up before you do it.