Buck Converter Design

I having some trouble determining the inductor value for a buck converter. Here are the design specifications:
Vin: 12.0V
Vout: 5.0V
Regulation: +/- 25mV
Load: 20W
Switching Freq: 200 kHz

Here is my work thus far:
R = = 1.25Ω
D = Vo/Vin = 5/12 = 0.416667
I_L = Vo/R = 5/1.25 = 4A

How do I find ΔIL so I can use the following equation to find my L value?
ΔIL = -(1/L)*Vout * (1-D) * T

 

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http://schmidt-walter.eit.h-da.de/smps_e/abw_hilfe_e.html