Just know that if the property you intend to get the HELOC on is a rental property, there are far fewer lenders that offer HELOCs on rental properties. 

Why not take out a HELOC instead and use that to buy another property or two?

How much more cash flow should I have above HELOC?

I have yet to see lenders that are willing to do a HELOC on an investment property and I am very surprised you were able to find some that did them in the past.

It rents for $3000 every month, and with an appraised value of $375,000 we refinanced using a 1st Lien Position HELOC strategy to pay down the balance MUCH faster while have equity accessible to take advantage of opportunities.

I currently do not owe a house, but my parents have thought about doing a HELOC (home equity line of credit) to help me buy a side-by-side duplex.

For my first property, I took a HELOC loan on my primary residence.

If you plan to use another kind of financing like a portfolio loan, or just cash from a HELOC on your primary residence, or whatever else then you buy within the LLC if you wanted.

Now that it's paid off, we opened a HELOC to leverage that equity for other deals.

In a matter of a few months, I had paid them off--used all my cash reserves knowing that I could always dip into the HELOC if an emergency comes up.