Thanks Eddy, that helps with my working out.

At 8 microsteps the resolution using a pinion with a pitch diameter of 23.87mm is 0.0149mm, and that is good enough for me ) But the holding torque is reduced to 19.5% peak value, per the linked paper. However, with a gear ratio 2.8, holding torque is increased to 54.6% peak value. It seems to follow when the gear ratio is include that 4 microsteps provide a resolution of 0.0106mm, with no loss of holding torque.