I had another thought, perhaps take the 2 TDS meters to an aquarium shop and then you can test them in some of their water or one of their tanks and then compare the figures to their TDS figure.
Do you have GH and KH test kits, it is probably worth getting one of each for this new tank setup, and to check the zerowater is still reducing everything long term as your water is so hard? I would test the zerowater GH and KH after about 50L, 75L and so on until the TDS reaches the stated 006, needing replacement? You would only need to do this for the first filter as the others will be the same! I know it is fine with my water but we are in a very small area of the UK which doesn't have such HARD water.
Simon