No maths. I use my e.c meter to get it back to the level the plants were happiest.
same here… my only question is if i need change this solution to a new, or i just can add water to make the water level and adjust the ppm with proportional fertilizer…
if i add 5L of watter, i take the GH chart, divide 5 / 3.8 = 1,315 then multiply this for the “recomended ml for a gallon”.
Great notice… i will travel more quiet :
and waste less fertilizer and water 
…