I figure whichever you’re more comfortable with is what you should use.
Because the exact cation and anion contributions in brewing salts are rough approximations at best, and even though measuring by weight is much more accurate, I believe measuring by volume is fine. You are dealing with rough approximations, no matter how many decimal places you use. You might be precise using a gram scale but no way it’s accurate without lab results.
I measure by volume, because I want ~50 ppm Ca in the mash and a little in the boil, and since the exact concentration is unknown, it’s close enough.
It’s kinda like the IBU. No way it’s accurate but X IBU calculated in my software of choice, on my system, tastes like *this* to me. It’s dialed in by a little more or less through trial and error over time.