I think Gecko reckon on 10 microsteps being the optimum for a CNC machine and steppers, so as close to 10 as you can would be the best bet. The Chinese drives often just have 2, 4, 8, 16......... so 8 would be what you want (or 1/8 as they often say)
So Ok you have 8 microsteps and 200mm motor steps (assuming its a 1.8 degree motor) so that means it is 1600 steps per motor rev.
Your ballscrew is 2.5 pitch so that in imperial units equates to 25.4/2.5 = 10.16 turns per inch so 1600 x 10.16 = 16,256 steps per inch. That of course is assuming you do not have any gearing between motor and screw.
Having home switches allows you to home and then you can set up soft limits and that defines the machines boundaries.
If you dont have home switches you can move to the negative extents of X and Y and positive of Z and press RefAll and that will set the machine coordinates to zero and that way you can set softlimits even though you dont have home switches. It does mean however that each time you start Mach you need to move to the extents and press RefAll. If you have home switches then pressing RefAll will make Mach move to the switches and back off until they are closed again and that will be set as machine zero.
Hood