Determining Alkalinity from pH and Lactic Acid

So i just had a go at backwards calculating the CaC03 content using the Bru'n Water spread sheet. I used the sparge water acidification tool. I added 1.25ml of lactic acid to 8l of water and the pH dropped from 6.8 to 2.98. This suggests the CaCO3 is roughly 10ppm.

Anyone care to critique my method? Ill test the results by brewing tomorrow and see how my pH predictions come out. Usually the spreadsheet is accurate when i start with rain water and add salts.

As background,: Im on rain water usually but had some bore water delivered recently which gave the water a subtle but noticeable mineral taste. I'd say my tank is roughly 1/3 bore water. The fact that i can taste the difference means there must be a bunch of other minerals floating around in there.