May I add a question? When you-all add lime to a potting mix, do you insist on dolomite lime? Back when I used it on lawns in New England, the "conventional wisdom" was to always use dolomite lime because the Mg it adds was needed, and all-Ca would be undesirable.
In most cases dolomitic lime would be used in a soilless potting mix to do exactly what you say, supply both calcium and magnesium. Both are essential secondary macronutrients but are not usually included in fertilizers. Some fertilizers do contain magnesium but one would have to check the label to make sure because I think most don't. Another option if not using dolomitic lime but where magnesium is not naturally in the potting mix ingredients would be Epsom salts (magnesium sulfate) but dolomitic lime would be a longer term solution.
The media pH in a soilless mix can actually safely be a bit lower than in mineral garden soil. Whether you need to supply both calcium and magnesium when adding to garden soil would depend on a soil test, but too high a level of calcium (or potassium) can result in less magnesium for the plants.