Backlash could initially be less if oil/grease dried up on the threads - although this would be a temporary condition. How used is the machine? You can get localized wear on the screws and slides if you did parts all the time in the same place on the table.
When checking backlash - make sure compensation is off before checking!
Also, it's summer - if your shop is hot and the last time you checked was in the winter and your shop was cold, you could be dealing with expansion...
Now .03mm = .00118", which is usually what the minimum backlash is, .01mm=.0004" which is a very small number.
Anyway, as to the "accuracy limits", generally you can work to .001" if you have taken time to adjust for backlash correctly and the z axis is in tram on both x (easy) and y (requires shimming). This assumes you haven't worn out one area relative to the others. You can also "map" the leadscrew - the manual has instructions although I've never bothered.
Also the stock Taig control is 8000 steps per inch, or .000125" per step - moving in amounts that are not divisible by that number often causes minute errors...
Finally, if there is sticking/jamming/lack or lubrication/poor gib adjustment you can lose steps.
See here for articles on tramming/adjustment: