I would suggest looking at your target market and running the rough numbers (50% rule, etc) on a number of properties. You'll get an idea of the cap rates in your area and what is a realistic goal. Remember, better deals take patience, marketing and/or fast action to find and get under contract. How quickly and how much effort do you want to expend to get a deal that meets your 20% COC goal?
Also, I second the idea that you must see what type of properties fall within your search criteria. Aiming for a 20% COC and using the 50% rule will typically turn up properties where the maintenance will be higher. You might need to use 60% for older, more poorly maintained properties.
Finally, how do you plan on financing the property? The conditions of the loan will greatly affect your COC and cashflow which are INVERSELY proportional as you adjust the down payment.
Things to consider. I wish you the best of luck. I'm in a similar boat now.