I used my Salisbury's 'basics' scales, which are undoubtedly ultra inaccurate! Don't forget that the whole principle of EI is that you add nutrients to 'excess' so you dont have to have too accurate when weighing out the powers.
Storage (have to use 'black' containers, or store in dark cupboards)
Using the solution: Because I was only using 1/2 litre containers, I couldn't dissolve the required amounts (had to shake like mad before dosing).
Mould! <- thus someone mentioned having to keep it refridgerated.... I'll let him tell the wife that then.
Poured all my pre-mixes down the sink in the end & now simply dose dry, straight into the tank using small measuring spoons.
I store my (litre) bottles in a dark cupboard: so far a month on and no sign of mold! I mixed the powders into smaller amounts of water first, then in to the larger bottle, and shook vigorously. I.e. I didn't add the powders to 1L of water straight off (which I guess would be hard to ensure it all dissolved like you say), but a bit of the powder to a bit of the water, a bit at a time