The 50% rule is the formula most used to initially assess a property to see if it is cash flow positive or not. It is:
Gross rents x 50% = Cash Flow before debt service (the 50% includes taxes, insurance, property management, utilities paid for by owner, maintenance, vacancies, etc).
Once you have that number, subtract your debt service payment (principal and interest), and you will have the cash flow.
Without the actual rents or your loan details, it's hard to provide an accurate example.
Now in your case, you'll be living in one of the units, so you need to subtract back out the rent that you could have received for the one unit you are living in. With a duplex, the net of this is you will have a monthly negative cash flow equal to your mortgage payment every month. For example, assume rents for each unit are $500/month – then:
$1000 month rents x 50% rule = $500 – $500 owner occupied unit = 0
The upside is on a $65K FHA financed property with the typical 3.5% down with today's interest rates, your mortgage payment will be far less than you probably would pay to "rent" a comparable place. Also, you will get the added benefit of mortgage interest write off, tax deductions for 50% of the property expenses, depreciation, and your other tenant will help pay down your mortgage, thus building your equity.
All in all, these reasons combined are why a lot of investors get their real estate investing career started by investing in 2-4 unit multi-families. If you can find a 3 or 4 unit multi where you can qualify for and afford the 3.5% down, you could actually be putting a some money in your pocket every month.
Hopefully this is enough to give you an idea of what you need to properly assess this deal.
NOTE: This is a formula only. You will need to do your due diligence and plug in actual numbers for the expenses to make sure this is indeed a good deal.