Several times you mentioned that the Huntington Botanical Gardens used calcium nitrate as a fertilizer. I would be curious to know what source of magnesium their plants use to make chlorophyll. Do they use tap water ?
I would say between 6.5 and 7. If, despite the addition of basic substances, you notice that the pH remains acidic (typically 5.6 or lower) , I advise you to repot in a fresh substrate. Your substrat is too decayed.
Materials that contain Calcium such as dolomite, crushed shells, coral ... should not be considered as sources of Calcium but rather as neutralizing agent of the the substrate acidity which develops with the aging of the substrate.
All this history reminds me that about 15 years ago I bought a Cattleya luddemaniana from a professional and paid as a botanical Cattleya. When it bloomed for the first time I realized that it was not what I thought it was.The real cattleya luddemaniana has small wings at the top of the column...
It is curious to note that I only need 50 to 60 ppm of nitrogen per week (and somtimes 10 days) to make large Cattleya to flower and that much more is needed for Paphiopedilum. Are they bad assimilator(s) ?
I am with Ray subgestion. I have also observed same situation on my Phrags. To get rid of this problem I treated with FloridaMix (a mix of Aliette and Dithane).
But as these materials are difficult to buy Bordeaux mixture is a good alternative. Copper concentration must be around 500 mgr/liter.
It seems to me that over time some growers are moving away from the nitrogen concentrations that had been recommended on this forum by Rick Lockwood. Was it insufficient? what problems have you encountered while feeding weakly but often?