It was trivial, on the order of a couple of hundredths of a gram. I did use a knife to carefully scrape the top level each time. Obviously that greatly increases consistency of measurement.
The OP seems very obsessed with precision here. Why do you need such precision when most people who aren’t using electrolyte mixes just add 2-3 pinches of salt (sodium chloride!!) to a bidon to bring it up to roughly the level of sodium that most people need to adequately maintain their sodium levels. Have you done a sweat analysis that leads you to the desire for such precision?
If cost is not an issue, my understanding is that Skratch Labs Hydration Everyday Drink Mix is basically just sodium citrate: Hydration Everyday Drink Mix - Skratch Labs