Here is the link to the energy need calculator on this site http://www.weightloss.com.au/weight-los ... ulator.htm
this will help determine your Basal Metabolic Rate or BMR
This will help you to determine how many cals per day you need depending on the amount of activity you do.
The theory goes to lose 1/2 kg you need a deficit in that week of 3500 calories. So you create a calorie deficit by 1) eating less calories than you require or 2) burning the calories with exercise. A combination of the two is the recommended way to go. I have found and I am sure Christina (Puffer) can back me on this it is not an exact science as the body doesn't neatly fit into a mathematical equation but over the long run it has worked out for me.
I have also read not to go lower than 1200 cals per day.
I currently eat around 1600 per day during the week and probably average 2000 per day on the weekend with the occasional 2500 on Saturday night but hey that's my night off.
My BMR is just over 2000 per day so I still end up with a calorie deficit at the end of the week, hence still losing weight.