I agree with Alla, it is more important to know what roots are experiencing than what you are pouring into. For example, there is a study of media acidification in Phalaenopsis (pH goes down after growing plants for a year or so). Even if the fertilizer is around pH of 5-6, the roots were experiencing pH of 3-4 (in sphag based media). But the researchers noted that even at the extremely low pH (almost like for Cypripedium actual), the plants weren't having problems. So, it is not known what would be the optimal pH for potted orchids. A lot of information about pH is coming from soil-based crop plants (which experience completely different root environment). The major part of acidification was also from the excretion of roots.
To measure the pH (and TDS) of pour through, one standard method is 1. fertilize (or irrigate) normally, 2. wait for 1 hour or so, 3. pour a small amount of pure water and catch the water coming out from the bottom, 4. then measure the pour-through water.
If you are going to keep truck of the long term change, I would standardize the amount of water you pour at the step 3 (e.g. 1/4 cp). Also I would measure the TDS of pure water before pouring and subtract the value from the pour-through value if your pure water isn't so pure.
With regard to TDS, lowest is the best if you are talking about the optimal. There is a range where plants can tolerate the concentration of solutes (plant roots can't absorb water if solute conentration in the surround area is too high). There is wide variation among species with regard to the tolerance. In any case, if you start with the pure water, you can add the mineral nutrients more.
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