calcium vs magnesium ratio and some sulfur.

[urgr8estfear]

High buds!
I've been doing some reading on ratio's between nutes.
About how too much of one can lock out the other.
In my case I'm a bit worried about my Ca vs Mg ratio.

I have three plants/pots 40 liters each.
Simple general purpose potting soil, I'm assuming Ca to Mg ratio OK here.
I know Batguano on leafman has a Ca to Mg ratio of 1 to 1 so that can't be a bad target for adding nutes.
https://www.leafman.nl/product_info.php?products_id=171

None of the nutes I use have Ca or Mg in them.
All three plants are over 200 days old and due to their age and flushing they've seen over 200L of water each.
I've checked with the water company and the tap water I use has a Ca to Mg ratio of about 12 to 1.
I calculated I've effectively added 7.2 grams of Calcium and 0.6 grams of Magnesium with the waterings.

I think the Ca to Mg ratio in my soil is going way off because of the tapwater I use.
I also calculated it would take 66 grams of epsom salt per pot to add 6.6 grams of Magnesium so that the Ca to Mg ratio of what I added is 1 to 1.

What I'm wondering about is :
-is the Ca to Mg ratio really that important? (something about antagonistic something)
-is it wise to add 66 grams of epsom salt to a 40 liter pot in 1 watering?
-if I add 6.6 grams of Mg using MgSO4.7H2O I also add about 10 gram of S. Please check the yellowness on my new growth, use the second link in my signature. Does this seem a good idea (too)? I'm not sure the yellowing is due to low temperatures at night or S deficiency. Air temps dorp to 55F/13C regularly.

Also nice would be to know what amount of Mg and S I can safely add, without burning the plants, to make absolutely positively 100% certain that there is enough Mg and S in the soil for the plant regardless of the low temperatures at night.
I'm worried about the Mg and S because no nutes I added contain them and I flushed the soil TWICE.
I also have my doubts on the stems/petioles turning red/purple all the time and now the yellowing.
Please have a look (signature 2nd link) and tell my what you think.

[chimera]

Epsom salt's recommended dose is 1 tsp per gallon(us) of water every 1-4 weeks, when I was growing in soil I used it like a half a tsp every 2 weeks, seemed to work allright. I did check out your plant(s) and they look fairly happy, but a pinch of epsom salt evey so often will help alot, especially during early flowering when you have explosive growth.