Caloric difference

Where you are looking at losing over 100 lbs a 2.5/lbs/week should not be a problem. One of the problems associated with quick weight loss is the loss of muscle. To counteract the loss of muscle I would encourage you to lift weights, if you are not already doing so, and to eat a lot of protein. MFP defaults to 15% but I would suggest more like 25% calories from protein to ensure you are maintaining as much muscle as possible.

Once you get down to 50-75 pounds to go, if not sooner, I would change it up to 2 pounds per week. Good luck.

I won't claim any credentials other than a brain (that my teenagers will tell you isn't nearly as infallible as it used to be), copious reading, and success in my own prior (and current) efforts. What you want to focus on is preservation of lean mass (assuming you want that). Base your minimal nutritional needs on the amount of protein necessary to sustain your current lean mass. To do that you need to measure your current body fat %. For the sake of argument, let's say it is 30% and your current weight is 400#. that would mean that 280# is lean mass (400-120).

Now you have to determine your activity level. This measure should be consistent with the setting you chose for the MFP BMR calculation (that you found to be 3550). What the MFP BMR scale lacks (at least explicitly) is a factor for exercise that impacts your target protein levels. You should use a factor that ranges on a scale of 0.5 to 1.0 for sedentary to elite athlete respectively. So something like this:
- 0.5: sedentary (no regular exercise)
- 0.6: light (some cardio exercise 2-3x/week)
- 0.7: moderate (regular cardio exercise 4-5x/week)
- 0.8: frequent (daily cardio and some resistance)
- 0.9: heavy (intense daily cardio/resistance)
- 1.0: elite athlete (intense multiple daily cardio/resistance)

Muscle mass needs more protein to rebuild/maintain based on activity level. A common formula for determining protein requirements is activity factor (AF) per gram of protein times the lean mass (LM), or AF/g * LM = daily protein requirement in grams. The formula gives us a foundation to build a minimum calorie content based on the most critical nutrient for maintaining you metabolic engine.

Now, let's further the example by considering lean mass of 280# with a moderate activity level. Your protein requirement to maintain and repair muscle would be 0.7g/lb and that comes to 196 grams of protein daily that should be spread across 3-5 meals. Using the macronutrient (carbs, protein, fat) distribution I favor (40%, 30%, 30% respectively) you can build a complete macronutrient and caloric picture. With a base of 196g of protein at 30% of calories (remember, carbs and protein are 4kcal/g and fat is 9kcal/g), total calories would be (196*4)/0.3 or 2,613. Carbs would then be (2613*0.4)/4 or 261g and fat would be (2613*0.3/9) or 87g.

So that's a long walk to get to the same number that MFP recommended (roughly, since I used a more rounded starting number and I assumed an activity level). Bear in mind that if you choose to consume fewer calories, you'll generally compromise something (individuality disclaimer applies here). If you consume too few protein grams for your lean mass and activity, you'll lose lean mass. If you consume too little quality carbs, you'll lack energy. If you consume too little quality fat, you'll burn through your carbs too quickly and limit your access to fat-burning hormones.

Basically, you need to pick a macronutrient distribution ratio that works for you and build up from a protein base that considers your actual activity level. You can customize your MFP goals to respect that distribution and then set your total calories using the formulae above.

Welcome and good luck.