This conversion calculator may help.
I am assuming this is a inital fill and not trying to raise from .020 to .030.
http://www.reefcentral.com/calc/unit.php
19L=5.01gals
IO is 1/2 cup per gal or .118L pergal=8TBLS=24Tsp
so I would say 2.5 cups or .59L
Mix it in a separate container/bucket. test it. adjust if needed and then add to your tank.
I am assuming this is a inital fill and not trying to raise from .020 to .030.
http://www.reefcentral.com/calc/unit.php
19L=5.01gals
IO is 1/2 cup per gal or .118L pergal=8TBLS=24Tsp
so I would say 2.5 cups or .59L
Mix it in a separate container/bucket. test it. adjust if needed and then add to your tank.