You can google "a4988 datasheet" and download the pdf and at page 9 it is written:
I TripMAX = V REF / ( 8 x R S )
Which means the max current (peak current or I trip current) is given by voltage on REF pin divided by Rs multiplied with 8. Further if you look at the schematic on pololu page you see that REF pin has a pullup resistor followed by a potentiometer. Basically that potentiometer on the carrier driver it is a voltage divider and playing with the potentiometer means the voltage on REF pin changes. That is what the potentiometer does and how the I trip max current is set.
So you need to take a multimeter and measure that voltage on REF pin, voltage named VREF. Modifying the potentiometer position will change the VREF voltage setting. If you can set that voltage to VREF = 0.18V, and you have RS=0.05ohms, then it will give I TripMax = 0.18 / (8x0.05) = 0.45A max. Given that your motor maximum is 0.45A and that perhaps the multimter also has an error especially at low values, then 0.16-0.18 or something like that should be your max setting, which is probably low, but thats what the math says.
Your motor with a 0.45A max is not ideal imo. If your motor would of had like a 1-1.5A max, then probably you could of afford a setting like 0.8A or so, which would of been different. With 0.45A max probably your going to have trouble with the torque and the motors will probably run quite hot.
I TripMAX = V REF / ( 8 x R S )
Which means the max current (peak current or I trip current) is given by voltage on REF pin divided by Rs multiplied with 8. Further if you look at the schematic on pololu page you see that REF pin has a pullup resistor followed by a potentiometer. Basically that potentiometer on the carrier driver it is a voltage divider and playing with the potentiometer means the voltage on REF pin changes. That is what the potentiometer does and how the I trip max current is set.
So you need to take a multimeter and measure that voltage on REF pin, voltage named VREF. Modifying the potentiometer position will change the VREF voltage setting. If you can set that voltage to VREF = 0.18V, and you have RS=0.05ohms, then it will give I TripMax = 0.18 / (8x0.05) = 0.45A max. Given that your motor maximum is 0.45A and that perhaps the multimter also has an error especially at low values, then 0.16-0.18 or something like that should be your max setting, which is probably low, but thats what the math says.
Your motor with a 0.45A max is not ideal imo. If your motor would of had like a 1-1.5A max, then probably you could of afford a setting like 0.8A or so, which would of been different. With 0.45A max probably your going to have trouble with the torque and the motors will probably run quite hot.