I simply do not have the education in water chemistry to understand that.
Oops. I've made the same mistake I have before and that is responding as if we were in the Brewing Science forum where I spend most of my time to the point that when I respond elsewhere I often forget that I'm not there. I would expect many, if not most, readers of that forum to understand what I posted and clearly some here do too as well.
I can only plug the numbers into the spreadsheet
That sort of says you aren't really interested in understanding further so I won't go into further explanation beyond saying that mEq/L is the natural way of expressing alkalinity. If you have alkalinity of 100 ppm as CaCO3 the lab analyst measured it by adding 2 mEq of acid to each liter of your water and then multiplied that by 50 to get the ppm as CaCO3.
When you add acid to the water you do so, in part, to neutralize the alkalinity. But you only need to neutralize 90% of it.
The strength of an acid is the number of mEq of acidity in each mL. For lactic acid of strength 88% by weight that is 11.8 mEq/mL which chemists write as 11.8 N.