I'm sure this is obvious to everyone else, but on the equation to determine the appropriate resistor for the photoresistor circuit, how is that the Vin is dropped from the equation. All the rest of the math makes perfect sense, but I don't understand why it is ok to drop the Vin.

The model equation is:

voltage difference = absolute_value((R*Vin)/(R+Rp_dark) - (R*Vin)/(R+Rp_bright))

and to find the local maximum ribs uses

F(x)=x/(x+Rd) - x/(x+Rb) - where x = resistance of second resistor, Rd = Rp_dark, Rb = Rp_bright

Notice the Vin is dropped from the f(x) function.

Any insight would be greatly appreciated. Thanks.