Hi. On a 6' Atlas I wouldn't take more than .010". On a CNC lathe you don't really care how many cuts you take because you fire it up and walk away, it does all the work.
The trick with a conversion like this is to get as close to zero backlash as you can. In a CNC machine backlash will kill you. As far as I know there are no anti-backlash nuts avaiable for the factory leadscrew so you have to make a new leadscrew with a thread that has anti-backlash nuts available. This setup has less than .001" backlash in both the x and z axis. WJS