I don’t think that we can’t find any magic formula or ideal ratio for nutrients. To much parameters are to consider from, Ph, light, CO2, water movement, interaction of nutrients, type of plants etc..
But we probably can lay down some guide line that will procured some satisfaction...
As Ca is involve, I can propose some ideas (only for low Kh, low Gh, acidic setup)...
• Ca is really immobile as a nutrient and one of the lower luxury uptake capacity (with magnesium)
Content in plant by dry weight: Ca=650 mg/kg
Ratio: +- (Ca) 3:1 (Mg)
If we transfer this in ratio in dissolution in water. With 40 mg/l Ca we have around 13mg/l.
• All alkalis have an effect on each other, to much of one will suppress the other
In order or influence we can put K in first (but Definitively PH seem to be the most influential external factor)
it appears that the reduction in Ca2+ absorption is closely associated with the increasing uptake of K+
The uptake of potassium, calcium, and magnesium ions by maize and the interrelationships among the cations have been investigated at 48 K: Ca: Mg ratios in culture solutions. Calcium was found to stimulate K+ and Mg++ uptake at certain cation ratios but inhibit it at others. Potassium did the same for Ca++ uptake, and Mg++ for Ca++ and K+. The uptake of Mg++ was generally enhanced by K+. The sum of the cations in the plants expressed in meq was fairly constant for treatments of the same K+ concentration at the low to moderate levels of K+, but at considerably higher (> 24 meq l–1) K+ levels the constancy was not dependent on K+ concentration. Potassium depressed, but Mg++ stimulated phosphorus accumulation. Calcium stimulated phosphate absorption at certain cation ratios but had no effect at others. The plant yield increased with increasing K+ up to 24 meq l–1 of K+ after which the yield tended to fall with further increase in K+. The yield was also increased by Ca++. Magnesium increased the yield at certain cation ratios and either depressed it or was without effect at others.
1— Definitively PH is important. Under 6.5 the Ca uptake will go rapidly down. 6.8 will be probably an ideal target when all the nutrients uptake are considerate. As is role in the cell as an acid neutralizing agent in conjunction with bicarbonate we can see that low PH, low kh setup have nothing to help the situation.
2— Enough Ca is needed in dissolution in the water, perhaps (if you look at my survey) between 40 to 60 mg/l should be right. But a little bit of Ca in the soil will probably a little help, if we have a healthy root system.
3 — Magnesium is needed , perhaps a ratio slightly higher on the magnesium side that the normally 4:1. Personally I will go around (Ca) 3: 1 (Mg) ex: 40mg/l for 13 mg/l (Kekon even say 2:1 work)
4 — Potassium is needed, 20 to 30 mg/l, as you say, Kekon seem alright for me. The only problem with k is to measure what we have and what is used. Try and error can apply, personally I will add around 16mg/l every week based on my survey and see.
5 — To much nitrate will be probably detrimental to Calcium uptake. Nitrogen tends to accumulate in the leaves and increase organic acid production, which increases the demand for Calcium to neutralize the acidity, if this calcium is in short supply calcium may be mobilized from the roots. This movement of calcium from roots is the demise of the root integrity and can lead to leaky roots and ethylene production signaling the plant to shut down in land plants. In submerged plants, I believe that nitrogen will have the same effect directly at the leave structure, taking away calcium for an healthy growth
Boron have a sweet spot in correlation with Ca, but I will play with that at last resort.
In my case I remember give an small overdose of boron in my survey, I have to look at my note what week it was...