Arduino current measurement
For maximum accuracy, you want 5 V output from the current-sense amplifier when 30 A is flowing through the shunt/sense resistor. Since the voltage gain of the device across the shunt resistor is 20 V/V, then the maximum required resistor value would be:

R = 5V / 20V/V / 30A = 8.33 milliohm.

You can see a close-by example in the second-to-last row of Table 1 in the datasheet that you linked to. For a 5 V full-scale output voltage at 25 A full-scale current and gain of 20 V/V, the calculated sense resistor value is 10 milliohm.

For best accuracy, you want to use the full scale of the ADC converter, that is generate 0-5 V for the current range of 0-30 A, which the resistor computed above would provide.