I have tap water that is very soft 1° dH for GH and KH (my water
treatment department reports it is around 30ppm or 1.6 dH, but I can
not get a reading equal or above 1°dH with my tetra kit).

As such, I generally add products such as Dupla KH+GH and baking soda
to help raise my GH and KH and to stabilize my pH.

I would like to start making my own mixture for KH/GH elevation and
plan to include the following:

CaCO3 (Calcium Carbonate or calcite - powdered lime stone)
NaHCO3 (sodium bicarbonate - baking soda)
MgSO4 (Magnesium Sulfate - epsom salt)

Has anyone done something similar? I am curious about the ratio to
use or if other chemicals should be used. I plan to use 1/3 of each
and then apply a measured amount of the end result to a gallon of
water and measure the rise in GH/KH and use that to develop a dosing
plan to apply when I change water.

If you have done something similar or have some input from experience,
I would love to hear the process and results.