Buck converter design for wide operating range
Normally you design for a current ripple ratio of 0.4, at the combination of input and output voltage, where the peak inductor current is at its maximum. For the buck, this is at maximum input voltage and minimum output voltage (or smallest duty cycle), assuming your load demands the same power regardless of output voltage. 

I can recommend the book Switching Power Supplies A-Z, S. Maniktala, on this matter.