SEPIC converter capicitors calculation
From the datasheet (on page 17, right above your \$I_{RMS} = 0.3 \Delta I_L\$ equation):


  The  input  capacitor  of  a  boost  converter  is  less  critical than the output capacitor, due to the fact that the inductor is  in  series  with  the  input,  and  the  input  current  waveform is continuous. The input voltage source impedance determines the size of the input capacitor, which is typically in the range of 10μF to 100μF. A low ESR capacitor is recommended, although it is not as critical as for the output capacitor.


This is basically saying: it depends.  If your upstream source is close by and decently stiff, a smaller input capacitance (~10μF) will suffice.  If the supply is further away or has a high output impedance, a larger capacitor (~100μF) is needed.

Similar to a boost converter, the input current is constant so the capacitors won't need to quickly supply a large amount of current.  This is opposed to a buck converter whose input current is discontinuous and thus a higher amount of input capacitance is desired, especially for high-output-impedance upstream supplies.

In my experience, designing input capacitance can be kind of an iterative process.  Put a reasonable amount (i.e. guess) in, observe the input voltage ripple (right at the input to the converter), and adjust as needed.