hey mate you're not alone! lol even though I'm British (Scottish really ) and i usually work in Imperial- i hate the metric system- and anything else European! lol
Isn't it enough I added Fahrenheit as well as Celsius?
Seriously... the advantage of mg/l is that it's approximately equal to the concentration of salt in parts per thousand (ppt) which is what you'd read on a marine biology paper quoting the salinity tolerances of a certain fish. For example here, for Hoplosternum littorale, where this fish is known to tolerate a salinity "up to 16 ppt".
So once you know that, you can move the sliders about on Brack Calc to determine that 16 ppt is approximately SG 1.010 at 25 degrees C, and that (remarkably) Hoplosternum littorale can tolerate water to that level of salinity, at least for some period of time.