Anywhere from 1.014-1.025, but it will be most productive at 1.014. Keep in mind that rapid changes of .005 SG will cause a failure to thrive and can kill the cultures.
You are much better off buying a single species starter culture. Our customers that try starting a live algae culture from our Phyto-Feast Live report that Nannochloropsis and Synechococcus typically win out. These two types of phytoplankton are very easy to culture. If you do wish to use our Phyto-Feast as a starter, just use a few milliliters. It is incredibly concentrated.