Normal sea water has VASTLY more calcium than carbonate alkalinity.  If you were to quantitatively precipitate the entire carbonate/ALK as calcium carbonate the calcium level would barely drop (like 15%) so there is no "equilibrium" in terms of there being any kinda of an equivelance between these two... which is why most people do OK with calcium and chase alkalinity. 

You use CO2 to form carbonic acid to lower the pH in order to dissolve rock (calcium) in the reactor, and you adjust the water flow rate and CO2 bubble rate to get the pH in the effluent stream to a level low enough to actually dissolve the calcium (say pH 6.5 is depending on substrate size type etc).  As well as dissolving the calcium through the pH effect, carbonic acid is simply protonated carbonate so your basically making alkalinity too... 

If you turn up the Co2 in attempt to get more carbonate buffer your going to drop tank pH as you turn the carbonic acid/carbonate/bicarbonate equilibrium towards low pH and your pH will drop at any given alkalinity so you cannot chase alk with a reactor without killing your pH... so thats why I recommend using Lime water... it will help consume the excess CO2 keeping carbonic acid formation a bit less, in turn help to raise Alk and bring more Ca.  I guess I am not sure I know if there is an exact ratio that can be relied upon in terms of delivery.  Ca once added is either deposited as coral skeleton or precipitates as rock or is in the water, because there is a CO2 gas / air exchange that is significant and that directly influences Alk I am not sure if its always the same system to system especially when you start pumping CO2 in with a reactor and your system has different gas exchange rates than the next one...

I guess I am not sure how much any of this can be predicted... I was just going along the lines of... we need more Alk... and Kalk is a pretty reliable way to do this and even recommended to help balance the pH issues with reactors...  Unsure really but what you say makes some sense as well.  Clearly you can use science to understand what SHOULD happen but we are all basically using a unique set up when you consider the number of variables that significantly influence these equilibrium so I kinda measure and adopt to what happens... in this case I THINK (I do NOT KNOW) that Kalk seems reasonable... its certainly testable and unlikely to hurt if approached with some reasonable caution and its about as cheap as it gets.

I will provide a very good link to a discussion on dosing Kalk and the use of Vinegar to increase calcium availability (first one below).  I also have a link to an article on alkalinity that is somewhat technical (it is after all chemistry).

http://reefkeeping.com/issues/2005-01/rhf/index.php

http://www.advancedaquarist.com/2002/2/chemistry

I will summarize the important parts.

There is an upper limit to the level of Calcium supplementation that can be provided through the use of Kalkwasser, but its perfectly acceptable, and even recommended, to use Kalk dosing as a method to both balance alkalinity and dose Calcium on systems WITH a calcium reactor as these typically drive alkalinity DOWN, which is what your seeing.  So... I think your in a perfect situation to use this.

However, if you dose to much kalkwasser to fast you can drive both Calcium and Alkalinty DOWN.  This is not probably an issue on a 300 g system but it would depend upon your daily ATO volume and if your using saturated Kalkwasser as ATO water, which is probably NOT required for you as your running a reactor all ready.

You can increase the level of calcium availability (reducing calcium carbonate precip upon addition to your tank or in the ATO reservoir) by dissolving the Kalk powder in vinegar purchased from the store as 5% acidity, its fine...  read the above first article to get an understanding of how much to use etc...  but tis about 15 ml/tsp of paste. 

BEWARE that you could theoretically accumulate enough vinegar to influence the alkalinity test result through titration of the acetate (vinegar=acetic acid = acetate plus a proton) which goes into your aquarium as this will titrate with the carbonate etc and likely influence most commercial test kits...

how much is open to debate.. two things for sure happen.... one balances this by consuming the acetate... 

bacteria consume acetate as a carbon source and produce two CO2 from a single acetate so it gets consumed and produces more CO2 to form appropriate carbonate buffering... and although the rate of its consumption is probably difficult to measure it likely increases over the first few weeks/months of use as the bacterial population expands with the new food source and then probably some sort of equilibrium would be reached, so its going to go away through feeding.

However, its relevant by addition because total carbonate alkalinity is titrated at about 3.5-5 mEq/L... at 5% acidity it would take roughly 500 ml (1/2 L) of pure 5% acidity store bought vinegar to the tank all at once to change the result by around 10%... at 15 ml vinegar per gallon of water with 1 tsp Kalk that would equate to 33 gallons of Kalk water... which I would go through in 1.5 months for my 70-80 gallon system so I am guessing this is no more than 3 weeks of addition so if NOTHING got consumed in a short time your alk tests would begin to rise detectably but in error...

So.. my advice is to start slow with kalk in mostly water and increase your vinegar additions to help calcium availability over a few months if your SUPER worried about it.  A LOT of people use vinegar and I have not seen anyone complaining about false Alk tests in practice but that means squat as no one tests for acetate.  I am almost certain that very little would accumulate due to consumption.  Hope this helps.... and is not too confusing.  Its like everything else, its complicated if you really want to understand it and it sounds confusing and even dangerous... but many many people find it very easy... and for the most part its trouble free so long as you go slow, monitor your tank, and dont dump 20 gal of saturated Kalk into your system in 5 mins.

I was in your store a couple of weeks ago for the first time. I only saw a younger employee working at the time and never heard about your current set up.