I am also faced with a similar dilema. I intend to put some Green Spotted Puffers in a 30g tank. You will have to perform a couple of simple mathematical operations to determine the correct amount using the ratio (established by the manufacturer) of salt/gallon in terms of specific gravity.
For example, 1/2 cup of the marine salt i bought per gallon gave a specific gravity (SG) of 1.022
I'm going to give you the mathematical operations i did for my 30g tank
The ratio (established by the manufacturer) is 1/2 cup per gallon SG 1.022
If i wanted full marine conditions i would need then 15 cup for 30 gallons at SG 1.022
7.5 cups would give me a SG of 1.011 for 30 gallons (half the SG since it is half the amount of salt)
3.75 cups would give me a SG of 1.0055 for 30 gallons
I needed between 1.010 and 1.005, so anything between 3.75 and 7 cups would be, THEORETICALLY, the amount of salt needed, given the specifications of the manufacturer (1/2 cup per gallon for SG of 1.022)
It is recommended to have a Hydrometer at hand to actually measure the Specific Gravity in the water
The link below has been very helpful to me, take a look at it.
http://www.pufferfish.co.uk/aquaria/water/brackish/faq/faq.pdf#search='brackish%20water%20faq'