Cash Flow to House Value ratio?

[Danny Ondik]

I've read that after calculating everything out, there should be AT LEAST $300 of cash flow to make a good rental property. But I have no idea if this for a $100k or $400k property? Is there some kind of ratio or percentage as a good rule of thumb that the cash flow should be on a SFH of the property value using conventional financing and 20% downpayment?

Thank you for any feedback and advice!