@SeymourGreen in general, you don’t want to change the calcium/magnesium ratio of your feeds, especially not for long periods of time. You are correct that if you add too much mag, it will antagonize your calcium and cause a deficiency. It is the same with potassium, too much K will cause Ca/Mg deficiency. Usually I would recommend supplementing Ca and Mg together, but also make sure you don’t add too much, or you might cause K deficiency (especially at the end of flower). K/Ca/Mg is a ratio that needs to be within certain ranges, because they all antagonize each other. 4:2:1 is generally a good starting point for me in hydro. I would consider 3:2:1 to be about the most Calmag you want to use before you will start noticing K deficiencies – but you can get away with a lot of things for short periods of time, depending on the stages of growth (if you know what the plant needs most and when).
However, if you are at the end of flower, I think the Ca usage is lower. Not 100% sure though.
It’s hard to compare dry and liquid nutrients in the same way. In dry form, nutrients are measured as % by weight, but liquids are usual % by volume. So you might want to use a nutrient calculator, or calculate ppms into the amount of your mixture first.