Reducing the space in my garage.

[JAZZCNC]

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 )

Nope you forgot the to include mr Pie.!! . . . .23.87*pi=74.989 /1600=0.0468

Also you shouldn't really think of Micro stepping in terms of increasing resolution. It has more affect making motors smoother.
If you need resolution then best to achive it by lowering the pitch.

Regards voltage then you can easily run those motors above the guide given on Gecko site. They easily run at 68Vdc without any trouble and the extra voltage gives that bit more torque and speed.
The amp you'll require will depend slightly on the type of PSU used. With Unregulated toroidal supply you'll only need aprox 60%-70% of total rated motor amps. IE: 4 x 4.2a=16.8 / 65%=10.92A