Alwaz use the same weighing scale to mark the weight loss, as every scale has upto 2 kg difference in caliberation. plus make sure you use a digital weighing scale.
women must make sure tht they are not nearing their menstruation, as every women gains around 1 -3 kg weight just before the cycles are due and this is absolutely normal. but it will not give you the right weight. so check ur dates before u check your weight.