batch water mixing???

[outtafocus]

I mix my water in a garbage can, I mix about 15gals at one time.
I used IO salt and tap water over 3 weeks ago. It has a powerhead and a heater running all the time. I keep it sealed and monitor the SG.

I just tested for pH and nitrate. pH was spot on at 8.2 and the nitrates read 0.
After reading this I will probably mix my water in smaller batches, but I am still happy with my system.

When I first mixed my Instant Ocean in my tank a few months ago, I followed the directions exactly : 1/2 a cup to 1US gal. My SG was so high the needle was almost off the scale on my hydrometer.
So I just do the add salt wait, test, add wait test, add wait test method.
Kind of a pain.

I have heard, that since salt is hydroscopic, it will absorb moisture from the air, and this will alter its weight and volume. So mixing by methods such as weighing and pouring x amount of cups is very inaccurate. Is this true?