TDEE - need help with understanding.

1240 is your Resting Metabolic Rate, or Resting Energy Expenditure. It wouldn't be considered your TDEE since you aren't going to be waking up and laying in bed literally all day, every day.

Your RMR comprises of 60-75% of your daily energy expenditure. Additionally, the Thermic Effect of food (energy needed to absorb food) requires roughly an additional 10% for most people. The third variable in TDEE is planned exercise which you would estimate energy expenditure for each exercise and include that number to the previous two. The last variable, unplanned exercise, generally is not included in the equation since mundane activity can vary too greatly. So you'd basically be concerned with RMR, TEF, and exercise calories to create a rough estimate of your TDEE.

To many people, MFP's methods are very confusing in setting the default deficit. The deficit, is the sum of calories reduced from TDEE according to eating less and upping exercise. So, a deficit involving eating 500 calories below TDEE is the same as a 500 calorie deficit where you eat 300 calories under TDEE and up activity by 200 calories. In fact, the latter scenario is the preferred option for various reasons, one of which is that being active leads to overall better well-being. Just make sure that if you include exercise as part of the deficit, that calorie restriction makes up the bulk of the sum deficit.

Honestly, the best way to estimate your true, present TDEE is to log food intake and exercise calories and see if you're able to maintain weight for a few weeks before making any changes to diet and exercise. The reason I prefer that people do this is because RMR and TDEE equations are predictions and can be off by 100, 200 or more calories depending on the individual's actual RMR due to chronic under-eating or diseases that impact metabolic functioning. Once you find your actual TDEE, then you can establish your deficit. Another reason is that many people may make drastic dietary and exercise changes before even knowing how many calories they truly need to eat to remain in maintenance. Without knowing your maintenance range, it really makes calculating for fat loss a lot tougher than necessary.