how much IO

The IO I have says to use 1/2 cup/gallon of water. When I use that I
only get 1.00021 after aeration and mixing for 24 hours. From what I
understand, that is not enough salinity. I think the correct mixture would
be about 25% more IO to get about 1.00025, wouldn't that be more appropriate
? Why would the IO package suggest using such a low SG ? Any other
product I have that requires mixing powder with water says to use too much,
I wonder why IO would be the opposite. Tell me if I am on the right track,
please.

pb