Meeting Future Income Goals from Real Estate Investing

[Brian Green]

I am a real estate investor specializing in small multi-family properties in the Saratoga Springs NY area. In the 1 1/2 years I have purchased 3 properties in the area and am managing them myself. 

I would like to know if anyone has a good spreadsheet or calculation method for determining how many properties(or a total market value of properties)  and at what rate or return I would need to purchase over the next 10 years to achieve a defined income goal? I would use my existing properties and their performance as my baseline and then extrapolate out from there but its a bit confusing with equity pay down schedules and factoring in future purchases over time. 

Say for example I want to reach $250k annually in net income from my investment business and I am realizing 25% ROI including equity earned from paying down the mortgages. I am trying to figure out how many properties(or their total market value) I need to buy and with what frequency I need to buy them in order to achieve my income goal ten years from now.