You don't need an R1 unless you want to make the math more complicated.
Now that I've studied the device a litte bit more, it is a constant current device. In the low state it will always allow up to 6mA to flow through it and in the high state it will always allow up 15mA. (both of these are nominal values.) Remove R1. I will refer to R2 just in name.
So you pick a value of R2 that makes sense for 5V.
Ohm's Law: V = I * R
5 = 15mA * R
R = 5 / 15mA = 333.33333 = 330ohm
Knowing there is a 330ohm resistor there when your device is outputting a "low" your voltage will be:
V = 5mA * 330 = 1.5V which should give an A/D reading around 300.
Essentially since voltage of the cell output is always the same, the voltage drop measured across a shunt resistor is always the same.. :-/ Not sure..
The current through the device is always the same. The voltage drop is relative to whatever resistor it is in series with. With the 330ohm resistor, the voltage across your device should be nearly 0V when it is HIGH. It'll be about 4.5V when it is LOW.