Salut
If it is an IDcnc premium, the screw is M10 x 1.5 so, for a setting = 1/8, the step per to enter in mach3 is 1066.66, for a setting = 1/4 -> step per = 533.33
To work in inch, just change the unit in the "settings" screen, don't change the step per or anything else in the config itself
If you set correctly the units in your CAM software, it will write the unit to use in the Gcode itself (G20 for inches) so you don't need to change them manually in mach3, when mach3 encouter the G20, it swith to inches.
be sure that config/select native units is set to mm (because the screw is in mm)
++
David