Oh sorry 80/20 my mistake 
but, yes 3lb, sounds much more doable 
crikey this is how I understand the science behind weight loss.
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Work out your weight in lbs
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Times it by 13
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The result is the amount of calories you should consume in a day
So if you're 13.1 that would be 183lbs
183 x 13 = 2379
This means that to maintain weight you should consume 2379 calories a day.
To lose weight you need to consume less than 2379 a day.
1lb is equal to 3500 calories. If you eat 500 calories less for 7 days you will lose 1lb. If you eat 1000 less for 7 days you will lose 2lb. Most of us usually fall somewhere in between
It's really important to track calories to be sure you know exactly how much you're consuming. Of course, if you go over calories, you will gain weight at the same rate.
To lose more quickly, you would have to exercise above your normal activity rate. Different activities burn calories at different rates but a brisk walk for one hour is about the same as a 30 minute run.
Once you start to tone up, your weight loss might reduce because muscle is heavier than fat, so it's a good idea to take measurements too.
Probably best to just weigh in once a week but if you have stuck to diet and not lost, maybe weigh again the next day just to check because it can be so demoralising if you've tried hard.
And when you do lose weight, you need to do those calculations all over again because, of course, your calories allowance will change each week.
Well, that's just about everything I know in a nutshell if it's of any use to anyone 