I have a Chinese 12 mm ballscrew on my S7 cross slide, single nut and IIRC the backlash I measured when first installed was about 0.03 mm, 30 microns. My Novamill has about the same value using double nuts on 16mm screws, but that has had a lot more use! On a lathe it's nice to have low backlash if you want to do profile turning though would have to turn compensation on. I have tried compensation on the screws before but it doesn't work very well if the backlash is too large. In fact I found it didn't work at all!