I read at Steve Hampton's website ( Aquarium Plants | My views, Steve Hampton, on how to succeed with aquarium plants ) that :

"Calcium can be added via Calcium Carbonate, Calcium Sulfate, or Calcium Chloride. Magnesium is easy to add with Epsom Salts which is Magnesium Sulfate. Aim for a 4 to 1 ratio and a 5-10ppm minimum for Magnesium."

So for my 600L tank, I added about 9 tsp of Epsom Salt to get 7ppm Mg. And to get a a ratio of 4 to 1 with Ca, I added 20 tsp of Calcium Sulphate; which gave me 28ppm Ca.

The fish looks okay (for now). This has been going on for about 3 days so the cupped leaves of my H.Difformis are yet to respond. But am wondering are all these within a safe and recommended limit? Please advise.

You'd only need about 2 tsp 2x a week of Epsom salt and maybe the same for Ca.

I really doubt your plants need more than that, and that assumes you are getting little from the tap as well.

Unless you are using 75 GAL of RO each week to do water changes........not likely.
Even there, blending with the tap to get a decent GH is wise.
Higher GH's are fine etc, but they are not needed.
All you do is add enough to rule out a limiting nutrient.

Regards,
Tom Barr