Dean
Several people (myself included) have posted on here saying that they have cleaned with various TDS readings going up as high as 20 ppm.
In my short experience I have found that once it leaves 0 ppm it goes up higher quite rapidly (I only have a small Di unit).
I have also found that you are more likely to 'get away with it' where the rinse water 'curtains' down the glass and less likely to 'get away with it' where the water 'beads' on the glass.
I'm looking to get bigger Di units as a result.
Regards
Mike