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The point of adjusting the motor current, is to find a minimal current level at which your mechanics function normally without missing steps or without any issues.
To get the best performance you should use the highest current that the motor or driver can handle without getting too hot. Higher currents give better microstepping linearity because the ripple current (which gives an offset) is comparatively less and higher torque gives less backlash due to friction.
I use Vref = 0.5V which gives 1.25A on a Melzi for 1.7A motors.