yup, that is the same (or very close) to the one i have been using here http://jdieck1.home.comcast.net/~jdieck1/chemcalc.html which i think is the same one in a different (flash) version. The funny thing is that in the calculator i used, it suggests raising the Mg, but the one you posted does not (all things being equal; Aquarium volume 35.8g, Ca now if 400, target 440, Mg now 1150, dKH 11), it says i am at my target Mg for that Ca and dKH reading.