I do pretty much as already suggested - I measure by added and removed volumes.
I hate buckets of water. I have carried too many buckets of water in my life. I am as married to the Python as to my wife. But when it comes to new setups, I measure as I fill so I get the actual working volume, but not exactly the world tightest data. These are fish tanks, not brain surgery. Once i know the working volume, I figure out how much I want out per partial and mark the corner of the tank. Python to that mark and I am happy, the fish are happy, and I don't have to tote any more buckets until I do another setup. To date, nobody has ever asked what those funny marks are on my tanks.
