The differences are small, like 1-2ppm, so in this particular application, they're basically irrelevant. Even if you're off by as much as 5ppm, if you're targeting 50ppm of Ca, I doubt being 10% off would make that big of a deal. I would do the math on it, but math isn't my strong suit. And who knows, maybe the oft-quoted 50ppm of Ca takes into account gravity of the wort.
It's a pretty esoteric question for homebrewers, but it's very relevant to chemists. Being off by a few ppm could be a very big deal to a chemical engineer. I mostly pointed it out because a lot of people say ppm = mg/L, so that's what I believed to be true all the time, in every circumstance, so it was extra-confusing when I realized they weren't always exactly equivalent.
I'm all for taking shortcuts and using approximations, but I think it's good to know when and where you're taking shortcuts. Good science is all about significant digits, and for brewers, I'd say they stop at the decimal point with regards to brewing salts.