Although some would say that this is over simplified ,it works.Because the calculated values will never match the standard values ,a very high degree of accuracy is irreverent. I use the 70% rule as that the capacitor supplies current to the load approximately 70% of the cycle .
C = capacitance in farads, I = current in amps, ?V = peak-to-peak ripple voltage, F = ripple freq in hZ
So putting it together we get C = 0.7 * I /(?V * F) for example for 4 amp at 40volt C=0.7*4/40*120=0.000583 Farads =583uF
Of course this is for zero load.
Unless you know what your load in ohms is you can't really calculate the ripple under load.