I tried ^shane^'s method and it works like a charm. I did see a nice setup at the NJ Hang this past weekend. Dylan used 1.75 Zing-it and made a prussik that loops back on itself after passing through the tie-out point on his tarp. The other end of the line was larks headed to his stake. For gross tension he used the stake as the toggle in a marlin spike hitch and then did the fine adjustment using the prussik. It was very slick. He also had a Spinn tarp which doesn't stretch. Might want to pm him just in case I'm not 100% accurate here.
Enjoy the elevation