Property Invesetment - Finding ROI

So I've been looking at buying a duplex or something and am trying to understand what is actually a good investment.  I put together a spreadsheet, here, that allows values to be plugged in and it will show the annual increase in value and %return on total investment in the property.

Plug in values in yellow and look at cells in green on the right for return.  Red shows my cost each month toward the mortgage.

It includes mortgage insurance automatically for down payments of less than %20 of the total value, and then stops them once the M.I. cutoff has been reached, configurable in yellow.  I'm not sure if I'm dong this right, but it looks good to me :p