If they had a rule of thumb, link between mortgage rate and investment return (eg. r-2 for savings accounts or r + 2 if you think you can beat the risk free rat

If they had a rule of thumb, link between mortgage rate and investment return (eg. r-2 for savings accounts or r + 2 if you think you can beat the risk free rate consistently), would you be satisfied? Increasing the rate of return gives you some interesting curves.