Minimum Rate of Return on a Note

[David Zickafoose]

I'm new to Real Estate Investing, and I'm still doing research to figure out what my niche is going to be.  I'm really intrigued with notes.  I was wondering what would be the minimum rate of return on note would be for someone to want to invest in?  Do any of you have a formalized decision matrix or multi-variable table to assist in your investment decision?  Do you look for different rates of return for different sized notes?  Thanks you in advance for your responses and taking the time to educate me on the process!