What I did with ARM was just continue to adjust the effluent ph, and measure effluent dkh at each ph. Going higher than 6.7 caused the dkh to really drop, and getting much lower than 6.6 didn't raise it that much. I'm sure you could do the same with any media. This was on one of the reactors I built; at the time I was building reactors and trying to get a really efficient design. By efficient I mean high effluent dkh at a given ph. What I found out was that circulation in the reactor is very important, that's why I ended up using Pan world pumps for them with bottom-up flow, and most important, the size of the reactor really matters. More reactor media and a higher reactor water volume means more contact time for the media in the acidic chamber, and better dissolution of the media.

I'm not trying to sound like a know-it-all with reactors; I built several pretty successful ones and was really into the design for a while. If I had a bigger tank I'd be using one again. IMO the biggest issue with calcium reactors for long term use is getting clean media. At low ph, toxic metals are much more soluble in water, and consequently any impurities in the media tend to get passed on to the tank. With KW, impurities in the powder and water tend to get precipitated out in the high ph.

If I were running a calcium reactor again, I'd take a good look at this new Schuran (sp?) media which is unfortunately very expensive but is supposedly very high purity.