Converting watts to temperature (deg C) is a thermodynamics problem, it can be done, but you are not giving enough information to solve the problem. You need to know the power dissipation per degree (temp) of your iron. Once you know that, you can (in most cases) assume a linear relationship in the absence of convection cooling or forced air cooling.

The temperature is based more on the type of solder being used than anything else. Use at least 100-200F over the melting point of your solder. At least 600F for lead-based solder, higher if it is RoHS-compliant (lead-free). Wattage is based more on what you are soldering (how much metal), etc. For dean's plugs, 60W is more is best.