how can i calculate decoupling capacitor value?
It's not for amount of noise. It's to supply current (the big one) and to react quickly (the small ones). So to size the big- consider current. Analog circuits, like sensors and stuff, would want 1uF and more. Digital- depends, but normally you put 0.1uF on each VCC input. For really fast stuff it would also be 1nF for low ESL. 

In most cases you will not feel a difference. When you will, it will take long hours to understand why the hell your circuit behaves weird way.