My goal is to buy a multifamily as a house hack by spring 2019, so I have been diving into Bigger Pockets and other resources to educate myself on cash flow analysis.

Lately, I've been working on clarifying my ideal metrics for buying the multifamily. For example, one of my metrics is that I want my payment/house hack rent to be 30% (or less) of my take home income. 

I've always used the 30% rule of thumb when renting an apartment, so it seems to make sense to also do it for owning a home. 

And that's where my question comes in. When are thinking about buying a multifamily, how do you set up this equation? Is it:

A) Mortgage+expenses /income = 30%

OR 

B) MY portion of the rent /income = 30%

I like Option A because it makes me feel safe: If for some reason I am unable to find tenants, I will be able to cover the full mortgage and expenses on my own. 

Using Option B opens up opportunities for me to buy larger/nicer multifamiles. But I'm worried that it could cause me to buy too much property and overextend myself. 

Does that make sense?

Perhaps another way of asking this is: How do I decide how much property to buy? Where's the line between good risk and stupid risk?