It's worth looking at a blended rate calculator before you make a final decision of the type of loan as it will show the new "blended rate" of your HELOC and first mortgage if you go that route.

Now the 9% doesn't seem to be a terrible deal, but then I calculated the risks involved.

@Ellen MarshallTo calculate a net monthly cost of $800 for a duplex house hack, input rent and property expenses.

Here's a bit more in detail about how rates are calculated for DSCR loans:1.

Calculate costs with inline meters on the electric and water.

I'm using the BRRR calculator to analyze a few opportunities. 

Hi Daniel, here is an easy to use rent vs sell calculator that may give you a better answer for what you are looking for https://www.crmproperties.net/rent-sell-calculator

Breakeven Analysis: I’ve read about calculating my breakeven occupancy rate, but I’d love to know more about how others in rental arbitrage set up and interpret this analysis.If anyone has a process or checklist for analyzing potential arbitrage properties, I’d really appreciate it!

There are stated or no income 2nd mortgage loans but typically these are fixed term and fixed payment type second mortgages meaning the interest rate usually doesnt fluctuate like a HELOC or home equity line of credit would fluctuate (prime + margin = annual rate calculated based on a monthly basis). 

I realized that the best way to compare and contrast these properties is to calculate the ROI of each.