Can anyone offer advice on ballnut quality/price. Ballscrew specification is easy to find - very little information on ballnuts which may be equally critical.

I can find this info on some relatively cheap ballnuts http://www.cnc4you.co.uk/resources/Ballnut+data.PDF

That would seem to say 20mm ballnuts have much better rigidity than 16mm and 20mm dia you have to load 25kg for 1Um movement? If true I would not need to look for better quality nuts - no mention of backlash though, is it assumed to be zero?