[I found this article while reading on specific gravity.
http://www.petsforum.com/personal/trevor-jones/specificgravity.html
Table 1 near the end of the article lists the "approximate specific gravity of seawater (S=35) for various standards".
(d4/4, d15.6/15.6, d20/20, d25/25, d15.6/4, d20/4...