I have hade a JP Superlightwind since 2009 wich I use at sea but not in open sea. With 7.6 it starts planning in about 5 m/s (10 knots) and with 9.3 in about 4-5 m/s.
If You have not been on the water for a while it could be a big thing to start with a 10.0 sail or bigger. Perhaps You will find about 8.5 suitable to start with? I had a friend about in Your level and he hade a hard time starting with 8.5, so do not get a so much bigger than that from start.