Ooh, the other thing I thought of (sitting on the toilet) was re: the accuracy of test kits. There is a general trend amongst aquarists to regard these things as absolutely golden, reliable, and they most certainly are not. If these things came with error bars, they would be horrible!
There are a lot of factors at play here. Typically you start by removing 5 or 10 ml of tank water. Do this with a syringe, do not trust the measuring vials provided as rather strangely these are often not so accurate (I would have thought that hard to get wrong, but there you go ...). If you are doing an important test like this, and I think for you this is an important test, get a reliable test kit, redo the test 3 times now, and repeat it for a few weeks, 3 times each time. The reason for doing it over a few weeks is to reduce an error there, and the reason to redo the test is that if these things are say 5% accurate (and they're likely not that good) then your starting point 400 might be 380 or it might be 420, and your end point might be 340 or 380. So your real calcium use might be from 0 to 80. Now that's the extrmes, but you get the point.
It's a lot of testing, but you need to get your benchmarks down pat.
Assess where you think the calcium is going. I see you have a clam, what else? Many people shovel in mountains of Ca, and most of it is precipitated into their sand bed.
Anthony Calfo has written about the slurry method, but I cna't find this on RC anymore. Google for his current forum and see if the article is resident there tho' beware chemistry is not always his strongest point. There are ususally useful articles by Randy Holmes Farley on reefcentral, and some in the online mag there that are quite good. And of course you should own Delbeek and Sprung vol3 for all the information on this, and numerous other topics that you will soon start to get into.
FWIW I am a moderate calcium use , and I use slurried kalk very early in the morning. I have some SPS, some LPS, and a lot of coralline
