I'm in the same boat - I used to run a calcium reactor filled with coral "rubble", but stopped this some time ago due to pollutants in the coral media. I'm currently looking for some good quality reactor media, but until then will continue to use DIY chemical additives.
So now trying to find cheaper additives for KH, CA and Mag.
I would suggest the following (using it myself...):
Drip limewater (Kalkwasser), using "BP grade" (human medical grade) slaked lime, available at any chemist
Boost calcium as needed, using calcium chloride (CP grade, or better but more expensive AR grade, obtainable from a chemical supplier)
Balance the added calcium chloride by adding either normal or baked "food grade" sodium bicarbonate (koeksoda), obtainable from any supermarket. Bake, or use normally unbaked, depending on the pH of your water.
Further balance the ionic ratios by adding a mix of magnesium chloride CP grade or better, obtainable from a chemical supplier, and "BP grade" Epsom salt (magnesium sulphate).
A full recipe and details on mixing, etc. can be found here:
http://reefkeeping.com/issues/2006-02/rhf/index.php
When using the food grade bicarb, what would be the general dosing per 100L volume?
Well, that would depend on your system's calcification rate. My corals use about 3dKH of alkalinity per week. With a total water volume of around 1000 litres, this results in approximately 12 teaspoons of baked bicarb and 130 grams of calcium chloride per week.
Hennie