Here's another scenario I deal with.I have a rough spot I fill my brute can to, then use a certain number of scoops. Followed by checking and adjusting.
After hearing stories about electronic salinity checks my final confirmation is doing a test on tank salinity and new water to ensure both have the same result on the same calibration cycle.
I have two 7 gallon buckets that I made 5, 4, 3 gallon marks on. I did them at the same time. I fill one with newly mixed saltwater and the other one I use for my waste water.
You would think draining waste water to the line would be accurate...it's not for me. My house floors aren't level and I add water to a sump in the kitchen, while pulling water from the living room. Sometimes I have to top off a bit more...and shut off the ATO just in case.