Here's the formula:
Hardness = Cal * 2.497 + Mg * 4.118
where Cal and Mg are amounts of calcium (Ca2+) and magnesium (Mg2+) in mg/L, respectively.
Mg/l and PPM are equivalent.
Mg++=(hardness-(Ca++ * 2.497))/4.118
I use 6 ppm, if the guys at the Brew Hut use 10, it doesn't make that much of a...