The L and C values are determined by Ton and Toff, which is a function of the switcher frequency, in this case the frequency is 1MHz..As BRE has suggested have a look at simple switchers, or look at the 78S40 data sheet...there are worked examples...Although I'm not sure if it will switch that high...TL494 is another...
Duty D = Vo_min/Vin_max = 20V/70V = .286
Von = Vin_max - Vout_min = 70V - 20V = 50V
L = (Von x D) / (r x Iout x f) = (50V x .286) / (.4 x 1.8A x 1Mhz) = 19.8uH
Thank you Smanches, 19.8uH for the inductance, where is the mistake in the formula I used?
And what about C value? Maybe C is not necessary (L and the load make a filter?