Your portfolio has cash flow positive that is what I would look at, not one property. Sure it is risky, but it isn’t like your approach lacks reasoning.
The math is very basic, applying it to your investment strategy a little complicated but it makes sense. The return on equity and “cash on cash” calculation just looks like a component of ROI to me,a substituion for takign the investmetn capital from somewhere else in your net worth. In fact it might prove to have been better just to take the down payment from your liquid assets, depending on what your returns are.Of course, you could always pay off the HELOC if rates go up.
Maybe I am seeing something wrong and you could say why using a very small portion of your liquid assets for the DP isn’t just as good?