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This is a freshwater planted tank forum.
And to answer your question, every brand and batch of salt is different so you can't measure XXXX cups of brand A salt of batch number B and have it be the same as brand B, etc, etc...
Once you use a particular type of salt for a while, you know approximately how much to add to get to a desired level, but you'll still have to use something like a hydrometer (yech) or a refractometer (YAY!) to get the exact levels.