Here is the mortgage calculator from bankrate:http://www.bankrate.com/calculators/mortgages/mortgage-calculator.aspxPut in 280000 for mortgage amount, 30 years term and say 3% interest (very low even for today), and the payment is $1180.49 for just principal and interest.

I use an App called Deal or No Deal Real Estate Calculator to quickly project ROI, Cash Needed and cash flow.

Figure out the average monthly rent for that unit, take out 35-40% in expenses, then subtract out debt and that's your cash flow.Then calculate the total cash flow annually for that property and plug in your downpay cash in and initial rehab done if any and you will get your cash on cash roi. 

Deal or No Deal Real Estate Calculator can help you with the calculations.

Have I done the calculation incorrectly?

I live in the Seattle area and I've been doing my own calculations on paper to see what kind of ROI would be possible.

Your current lender is your best bet.

You did save the 21k pretty quickly if I recall, so if this is attainable in your area you could just get another one pretty quickly.Have you already done each of those calculations?

Essentially since this is so highly leveraged you would have a much lower IRR that a more nomally leveraged property, and you could not calculate cash on cash at all since there is no initial cash investment.

If you don't understand how to use a financial calculator, I recommend you seek out Gary Johnston.