A4988 - setting Vref

[ProBUser]

Can someone explain why Geeetech says to set the Vref on the A4988 driver to 1.0V.

All the drivers on my Pro B acrylic have 0.2ohm (R200) sense resistors and the supplied NEMA17s have a rated current of 1.5A so doing the maths, Vref = 1.5A x 8 x 0.2 ohm = 2.4V, which is outside the adjustment range of the driver trimpot. Regardless, setting Vref to the recommended 1.0v has worked OK with the original MK8 over the past few months.

However, I have just upgraded to an E3D Hemera but the extruder is simply clicking and not pulling the filament through. The Hemera stepper is rated at 1.33A and their instructions are based on A4988 with 0.05 ohm sense resistors resulting in a Vref = 1.33A x 8 x 0.05ohm = 0.532V. Both steppers are 1.8 degree 2-phase.
Dave