Few people know that there are mathematical forumulas for how your body burns calories--using them, it is quite easy to tell just how much you should eat!
The first steps will calculate your Basal Metabolic Rate, or BMR. Your BMR is base number of calories your body needs each day to just keep you alive--breathing, heart beating, etc.
Be honest. The results will depend on you providing good information. Treat this like a doctor visit, not your driver's license weight. Using the tools correctly is how you can get to that driver's license weight!