Advise on how to calculate ROI?

[Zach Wilson]

I'm purchasing a house within the next 4-6 months. I have a friend that wants to move in and rent from me while I'm also living in the property. I'm curious as to how I should calculate the ROI? I've used the "Four Square Method" for regular property's with tenants, but not for one while I was living in the house and only renting out a room. Looking at the math it doesn't really look like an opportunity that would provide any cash flow, but instead just provide a way to make my mortgage payments significantly cheaper every month since that's what I would put the rental income towards. Thoughts?