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A good starting point for the current is 0.7 times its rated current. This is typically ~1A with the recommended 1.68A NEMA17 motors and that is about the maximum current the Pololu can deliver without a heatsink or a fan. Note that the rated current of a motor is usually that which gives an 80C temperature rise, which is too hot for plastic brackets, hence the reason to under-run them.

The recommended way to set Pololu drivers current limit is to measure the voltage at the Vref test point. The A4988 datasheet gives all the required information to configure the current limit of a Pololu driver :

The peak current I=VREF8×RS

where RS is the resistance of the sense resistor (Ω) and VREF is the input voltage on the REF pin (V).

On the Pololu driver board, the RS value is 0.05Ω and the VREF can be measured on the test point shown below, or the metal wiper of the pot. I is equal to the recommended current limit for your stepper motor multiplied by 0.7. Thus you can determine the VREF you'll need with:

VREF=I×8×0.05=0.4×I

For example if your motor is rated 2.8V at 1.68A, you adjust the pot so that you measure the following value for VREF :

VREF=1.68A×0.7×0.4=0.47V