I am unable to find actual amounts of Mg and Ca to add to my plants of which I have hundreds (miniature and standards) My fertilizer does not have them added. I was adding a tsp of a mix of the two (ca carb and epsom salts) as a side drsg and just about read adding a Tbsp Mg/gallon to weekly watering but it didn't mention Ca. I use an osmocote as well as bi weekly water soluable fertilizer.
Another question. I thought I have botrytis but it is always referred to as black or dark spots. What I have looks like water spots which appear transparent or grey on white petals. I do get some control using physan. Is this likely still botrytis?
With regard to your first question, Ca/Mg is absent from a lot of plant foods because that is usually present in the water. You need only be concerned about it when you use RO water, or rainwater. If the latter is true, there is a specially formulated fertilizer containing the proper proportions of Ca and Mg. It is marketed as University of Michigan fertilize or as CAL-MAG. Be sure to get the CAL-MAG formulation for RO water.
In general, orchids require little fertilizer. It sounds that you may be over-fertilizing. Salt buildup In the potting mix can lead to root damage. Also, the Osmocote can promote potting mix breakdown so I recommend only 1/2 tsp per gallon in water of CAL-MEG, or your regular fertilizer, per week. To clear out salt build-up, you can use a solution of Epsom salts once per month.
Botrytis is a fungus appearing on flower parts that get wet during watering. It usually occurs overnight as that is when humidity is highest and evaporation is lowest. Prevention is the only effective strategy. What helps prevent the spread is less watering, water only in the morning, improved air circulation, and the use of a fungicidal spray. The latter can prevent the spread to unaffected areas.