You should not be measuring the power supply current to set the current limit. As the note on the product page says:
Furthermore, your calculations don't seem correct. The full-step current is 70% of the current limit, so to get a full-step coil current equal to your motor's rated current of 1.8 A, you would have to set the current limit to 1.8 A/0.7 = 2.6 A. However, this value is only really relevant if you are using it to calculate VREF; if you are setting the current limit by measuring the coil current directly, you should just adjust the potentiometer until you see a measured coil current of 1.8 A.
You will need a heatsink, and possibly a fan, to keep the driver cool enough to deliver more than about 1 A. We do not have any specific heatsink recommendations.
Since the torque of a stepper motor depends directly on the current it is driven with, you will not get the rated torque from your motor until you set the current limit on the driver to allow it to draw its rated current. Microstepping and the motor voltage should not make much of a difference.