It's funny that you ask given the position the 50 gal bag of IO Salt has taken in the "I'm tired of paying full retail" thread, but...on the back of the IO 50 gal bag, they say to mix 1/2 cup salt per gallon. Personally, I'd take this as a good approximation but still measure the salinity.
I've found that consistently, 3 cups of IO gives me specific gravity of 1.023 at room temp (78F or so), in 5 gallons of water. I think the bag says 1/2 cup per gallon, I usually start with that and add another half cup once it's dissolved and aerated.
I'm with Jenn - I mix 3 1/2 cups (a little more than they recommend) in six gallons for 1.023. But as a general rule start with 1/2 cup per gallon but make sure and test the water before you add it!!!!
