Here is my reaction which is more of a gut reaction than anything that is researched and validated: it seems that you are over-complicating the whole BMR thing. At one point in my life I was about 75 pounds overweight. I looked up how many kcals were generally needed to maintain a person with my frame, I ate a little bit less than that number and put myself in a calorie-deficit situation, and then Bob's your uncle.

Therefore, I agree with mc that BMR is a rather low priority but once you do consider it, it really is not that difficult. Your Kcal maintenence amount is the same whether you have 20 pounds of blubber or 200.

Again, that is my unlearned take on the situation.