How to calibrate a load cell?
The first thing that about Load Cells that always comes to my mind is the circuit which amplifies the load cell's signal, Usually the load cell signal changes in a low mv range, so you have to put a high gain circuit in the way of the signal, which amplifies signal and NOISE. You have to use an instrumentation amplifier for this purpose, otherwise, your signal would become so noisy.

And try to implement a low pass digital filter too, it should compensate the fluctuations and would give you smoother signal.