Actually, if I understand the process correctly (which I might not have it all correct) using your Basal Metabolic Rate isn't the right thing to do with medifast.
The process that our bodies use when we are following the 5 and 1 plan on medifast is ketosis. Since using the 5 medifast meal replacements gives our bodies the prefectly formulated balance of nutrition (protein, carbs, fat and other nutrients), our bodies enter a light state of ketosis (fat burning state). Once our bodies are in this state AND we are getting the perfect balance of nutritious foods, at 900-1100 calories a day, we burn fat. Its not so much just the fact that we are taking in fewer calories, but we are taking in the right kind of calories that allows our bodies to burn just fat.
Also, drinking lots of water helps to cleanse our bodies of the by products of this process (and the junk leftover from the shrinking fat cells).
I hope that I got this right, if not, hopefully someone can correct it. I do know that we can't just go with the BMR as a reason for our weight loss.