For those of you that purchased the Hach test kit. I strongly recommend that you dilute your tank water sample prior to testing it or you will find that the end point is hard to see, and you will also end up using A LOT of titrant, especially for the magnesium test. I have found a good dilution to shoot for is 1/7x the original concentration. I use 1 part tank water to 6 parts DI water. I then use this diluted sample for BOTH the calcium and magnesium test. Then I run the tests as the directions suggest.
Rather than adding the titrant of the kit dropwise however, I have found I could get more accurate measurements by adding the titrant using a 1 ml syringe with a needle attached. If you use this method then first test for calcium, then multiply the milliliters of reagent you used by 854 to get ppm of Calcium. If performed carefully you should be able to get resolution of the test down to about 17 ppm. Your tank should be >400 ppm Calcium
For the magnesium test I also use the diluted sample. To get the final magnesium value in ppm use the following formula:
[X ml of reagent * 2144 - (y ppm calcium/0.4)]/4.7 = ppm Mg
where x is the ml of reagent used in the magnesium test, and y is the ppm of calcium determined in the calcium test. Your tank should be >1250 ppm Mg, best about 1350 ppm.