If salinity is way off just keep adjusting your dosage over a few trial and errors. Rather than filling the whole bucket at once, just try like a half gallon of fresh and add measured salt until it is perfect. Then just increase dosage proportionately to the amount of water used.
I don't mix my own water. I get 10 free gallons of SW each time I spend 10 dollars at my LFS <3. And besides, a RO/DI unit isn't really worth it when working with a 24 gallon tank.