No need to add calcium to a vinyl pool...ever. Unless, of course, you feel the need to be charitable to your local pool store and buy useless stuff to keep them in business.

As mentioned, a good way to add calcium is by using Cal-Hypo to chlorinate. Look for 63% or more effective Chlorine--you won't find it in HTH products anymore--they are less than 50%-- but some of the pool stores private label it. Like Sun Pools in NJ here has a private label 63% Cal-hypo.

If you can STILL find the WalMart HTH 5-way kit, GREAT! It was the VW Beetle of good test kits--$15 gets you most of the tests you need, only the chlorine test is limited to 5ppm max. But, with the CarlD ShotGlass method you can get readings to 15 or 20ppm. Simply mix a shot glass of pool water with a shot glass of steam-distilled water and you double the scale (to 10ppm). with 2 shots of distilled to 1 pool you triple it, etc. but you lose accuracy.

Unfortunately, Arch Chems seems to have decided that such a useful kit is too much information in a home-owner's hands and is now pushing "6-way" test strips. I haven't seen the 5-way in the local WalMarts in 2 years--and it was the only reason I go to Wally-world.