I use a Brute food-safe garbage can with a powerhead & heater to mix mine..... I keep about 24 gallons mixed at any given time in case a large change is needed or I need to set my QT up in a hurry.
If you're going to err one way or the other, it's better to mix your water a little on the higher salinity side, as it's very easy to dilute it down a bit. If it's on the low side, then you have to add more salt & wait for it to disolve again. I've got mine down to where I know 0.3lbs/gallon will nail 1.026 every time.
Distilled water is better than tap, but not ideal..... you really want to either find yourself an RODI
source or get your own unit. I've purchased supposedly "distilled" water from the store just out of curiousity, and have seen TDS
readings upwards of 50-100..... NOT good. A TDS
meter is cheap insurance to see what's really going into your system.