I think the key to this is knowing your true BMR (Basal Matabolic Rate) - the number of calories you need to take in to maintain at the weight you are if you were totally sedentary - your calorific output, a continual measurement, so that you know how may calories you are burning, and then an accurate count of the calories you are putting in.
Without knowing these three accurately - and we all differ a little on the BMR - you cannot calculate your rate of weight loss.
You would have to work out what calorie defecit you were running at (BMR + CaloriesBurned - CalorieIntake) = CalorieDefecit. For example, if you have a BMR of 1500, and you burn off 1000 calories a day, and you eat 500 calories, then your calorie defecit would be (1500 + 1000 - 500) = 2000 calories defecit. Once you are in ketosis your body gets that by burning fat, and 1lb of fat gives your body 3500 calories. If you are running at a 2000 calorie defecit, then your body will burn 2000/3500 lbs of fat a day (~ 0.57 lbs a day, or 4lbs every 7 days).
Now, you need to remember that your calorific output will not be the same every day, and as you get lighter your BMR decreases, so 2 out of the three parameters are constantly changing.
You can estimate this, but you would have to measure your BMR just about every day, and constantly wear a HRM to measure your calorific output. What a pain!
I say just work out your average weight loss (ignoring first week) and take it from there. Doing it that way means your end date keeps changing a little, but thats no different to doing it the accurate way .... and its far less hassle.
My spreadsheet under the stickys does it for you.