How would one go about avoiding that in a tap water scenario?
By far the most common scenario would be a seasonal change in the calcium content (usually CaCO3 derived from limestone, which will change both your dKH and dGH). As an example, where I live the the municipal water is a mix of ground water and surface water from nearby lakes (we got a lot of relatively clean lakes in Minnesota). The ratio between lake and ground water changes over the year depending on rain and snow fall throughout the seasons - so sometimes, such as dry summer months the municipal water gets quite a bit softer due to a higher ratio of lake water which are usually quite soft. For me personally, it's not an issue as I rely on 100% RODI water.
If you're relying on 100% tap water all year around and you see a
significant seasonal TDS increase the only remedy would be to blend in RO or rainwater. Conversely, if you see a significant decrease you may want to add additional minerals such as Calcium, Magnesium and possibly Nitrate (if you are partially relying on the tap water to supply NO3). If you're using partial RO and partial tap water you may want to change your RO to tap water ratio accordingly.
While the TDS meter will give you a hint that things changed, you can only determine what actually changed by measuring individually (with a dKH and dGH and NO3 test kit), Knowing both dGH and dKH you can derive the Ca and Mg content fairly accurately.
Assuming all Calcium is from CaCO3 and content of Iron and other trace metals that would impact the dGH are insignificantly low.
dKH x 7.12 = Ca ppm.
(dGH - dKH) x 4.34 = Mg ppm.
For instance, if you measure 10 dGH and 8 dKH.
The 8 dKH would most likely correspond to 57 ppm of Calcium from CaCO3 (57 / 7.12 = ~8 dKH)
The 10 dGH would most likely correspond to the 57 ppm of Calcium coming from the CaCO3 plus an additional 9 ppm of Magnesium (plus some insignificant trace amounts of iron etc.) such that 57 / 7.12 + 9 / 4.34 = ~10 dGH.
Note: in the UK its common to have very low levels of Magnesium in the tap water so the dGH will be roughly identical to the dKH.
Cheers,
Michael