Comparing pounds lost between two people is a bit of an exercise in telling you not much. Unless you weigh exactly the same amount, have the same amount of exercise, eat the same things and have body metabolism that's exactly the same thing, along with the same family history inclinations, it's like comparing apples and giraffes -- it doesn't really compare well.
For instance, if two people lost 20 lbs in the first 30 days, you'd think their "success" rate was about the same, right? Well look at this:
Person 1 weighed 180 to start.
Person 2 weighed 325 to start.
Person 1 lost 20 pounds which is 11% of their body weight lost.
Person 2 lost 20 pounds which is 6% of their body weight lost.
See how using pounds skews the perception? With percentages, even though we know we shouldn't compare with each other, it gives a better view of the amount lost -- puts it into better perspective.
To calculate percentage of your total body weight you've lost:
Starting Weight minus current weight = total pounds lost so far.
Total pounds lost divided BY your starting weight, then move the decimal 2 places to the right gives you your percentage lost.
An example: Someone with a starting weight of 240 who now weighs 219:
240 minus 219 = 21 lbs lost so far
21 divided by 240 = 0.0875;
to get the decimals in the right place, move the decimal over 2 places to the right = 8.75%