It is only an estimation, but if you use my spreadsheet (under sticky Graph/Spreadsheet thread) the newest version estimates your BMR (Basal Metabolic Rate) for your current weight. This is the estimated number of calories your body needs to remain at the same weight when sedentary. If you take this figure and work out your calorie defecit you can estimate your weight loss once in ketosis.
For example, lets say your BMR is 2000Kcal, and you are taking in 500Kcal a day, that means a calorie defecit of 1500Kcal per day. 1 lb of fat equates to about 3700Kcal, so 3700/1500 gives approximately 2.5 days to lose a pound, or around 3 lbs a week .... which is just short of a stone a month. Remember, BMR is what you need if sedentary, and none of us are which means that your calorie defecit is greater, so the weight losses will be a little faster .... hence the stone a month being achieved by virtually everyone.
The heavier you are, the higher you BMR so the greater your calorie defecit and the faster your loses. As you get lighter your BMR comes down, your calorie defecit decreases, and so does the rate of weight loss.