I used to think it was a major pain mixing up the water...now it takes me a few minutes. I'd encourage you to keep trying mixing it, it'll really save you lots of money.
Buy some 5 gallon buckets and mark them with a marker, so you always fill it up to the same amount, and then get a REFRACTOMETER. It'll let you get very accurate readings on your salinity.
Now that you're always adding the same amount of water and have a very accurate readings, you can quickly learn the precise amount of salt you need to add for a given salinity and it makes it much less of a guessing game.