[ataxy]

you backlash is the amount of play the scew will have when you reverse your movement so lets say you go x-1 then x1 the screw will turn slightly before it engage in the opposite direction as an example you will get maybe on a brand new acme screw about 0.005 of backlash this would mean you would go x-1 and actually x0.995 and not x1 to check backlash on an a x2 i would check it at three point on the table with and indicator so both edges and themiddle and do and average but remember contrarly to ballscrew acme will wear faster because of the friction and in the case of circular interpolation like for round pocket backlash compensation does not a good job but on linear motion its not that bad but still not as good as ballscrew so if you use it recheck often to compensate for the wear of the screw

ps.: also remember that you wont be able to drive your motor as fast with acme as stepper tend to loose there torque at high speed wich raise the risk of loosing step wich is not a problem with servo where the encoder will make the servo compensate for the lost step