Figure out how many hours you can work in a month. There is no standard here. In the U.S. the standard work day is 8 hours but you may only really be able to focus on coding for 4 - 6 hours per day. Multiply that number by 12 and then subtract the number of hours you will lose from vacations, sick time and any other down time you might have. Then divide that number by 12 to get your monthly average. Next come up with your preferred monthly income plus business expenses. Divide that by your average monthly working hours. The resulting number is your hourly rate.
However, don't let that be a constraining number. Depending on the client, circumstances and other variables, sometimes you can charge whatever you like. In a scenario where you are relatively busy and you get a new big client asking for you to take on an emergency project, then go ahead and throw out whatever crazy number you think the client might accept. You already have work, so it doesn't hurt if the client declines.
Also raise your rates as you get more clients lined up and you get busier. That's supply and demand. As your demand goes up and you have more opportunities than you can handle, raise your rates to bring that demand down to a level you can handle.
Your rate shouldn't be based on where you live other than the fact that you need to at least charge enough for your living expenses, and that can greatly vary based on where you live.
