i think you go wrong way with the limestone
0.14mS out of the tap is soft water. most nutrients are formulated for hard water (0.4-0.5mS).
bugbee's method is recirculative hydroponic. you need to get the calcium diluted in the soltution, not in the medium.
what you need to use with your soft water is a soft water formulation
for exemple ghe floraseries "soft water".. ionic soft water
personnaly i work with pure RO and my favorite line is "dutch pro hydro/coco RO/SO"
i mix it around 2.2mS with RO and diretly get ph 5.8
soft water nutrient lines contains more calcium/magnesium and also they are formulated to not bring the ph down
the 20-10-20 bugbee method is not for recirculating hydroponics. he wrote a paper on that in 2004 and its way different than the 20-10-20. he specifically states u should be watering to 10-20% perecent runoff. u can keep the intended rations without leaching the 10% off but it gets "tricky" in his words. and when u start to break down the 2004 paper u can see why.
the 20-10-20 he reccomndeds from his video has nothing bestides NPK in it. in almost any scenario i think ur going to need to add calcium magnesium and sulfer. hard tap water can come in around 80ppms i think. u would want something closer to 150ppm throughout the grow. so in an area wth soft tap water... lets say 40ppms. ur going to want tio be adding even more of the secndary nuitrents.
my reccomndations would be to grab some epsom salt (mag and sul). then some kind of cacium to wrok with the fertalizer u already have without chnagin the ratios too much. so i wouldnt use calcium nitrate cuz to achieve the amount of calcium u will need would push ur nitrogen level too high.
gypsom is calcium sulfate. that would work fine in either the medium or the feed. i put it in the medium to bugbees specs but ill also add to the feed slighly
calcium chloride is another option. u could use those two or a combo of those two w ur 20-10-20 to get to the calcium levels needed.\
might be easier to get a 10-10-20 and use calium nitrate. when u add the calcium nitrate it shoiuld get u to addequate calcium and adequate nitrogen