This is what the Pololu page says - it seems very straightforward.
One way to set the current limit is to put the driver into full-step mode and to measure the current running through a single motor coil without clocking the STEP input. The measured current will be 0.7 times the current limit (since both coils are always on and limited to approximately 70% of the current limit setting in full-step mode).
Another way to set the current limit is to measure the voltage on the “ref” pin and to calculate the resulting current limit (the current sense resistors are 0.100Ω). The ref pin voltage is accessible on a via that is circled on the bottom silkscreen of the circuit board. The current limit relates to the reference voltage as follows:
Current Limit = VREF × 2
So, for example, if you have a stepper motor rated for 1 A, you can set the current limit to 1 A by setting the reference voltage to 0.5 V.
The reason for the 70% is because if you use a multimeter to measure the coil current in a stationary motor set at full-step mode what you measure will be 70% of the limit that is applied when the motor is running. The driver lowers the current when there are no steps being made.
Be VERY CAREFUL never to disconnect the wires between the motor and the stepper driver while the driver is powered up. The driver will be instantly destroyed,
...R