Ct clamps accuracy and calibration

My (Current cost) ct clamp seems to be reading quite a lot higher than the real power ( as recorded by current cost pulse counter reading device.
It seems consistently 30% higher. I have “solved” this by adding a thin piece of paper between the clamp jaws ( reduction is dependant on thickness of the paper and I only needed quite a thin copy paper piece to correct the reading. Worth a try I reckon.