Are you a flight simmer? If so, you can use your method for getting some kind of flex/assumed temperature. For real aeroplanes it is more complicated as obstacles need to be accounted for. Pilots aren't normally involved in this process, they typically have either paper charts or software in the cockpit that has been put together by performance analysis people.
You can almost do it all from information provided by the flight manual but you would need an obstacle chart for the airport so you can work out the climb gradients required.