calculating Vref for A4988

What is the formula for calculating the Vref for an A4988 pololu driver.?
Is it Vref= current per phase(I)/ 2.5

or is it Vref= (curent per phase/.7 ) /2.5 {in case of full steps}

I am also having problem of layer shifting

The formula is:

Vref = 8 * I * R

where I is the peak phase current you want (normally 50% to 80% of the rated phase current of the motor) and R us the value of the sense resistors. R will be either 0.1 or 0.05 ohms depending on your drivers.

I prefer to use all-in-one controller boards that let you configure the motor current in software.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].