I do a lot of work vinyling roofs and heres what I do with the shark fin.
Use one piece to wrap one whole side of the fin, and use a heat gun to heat and stretch it across the back flat portion as well. Then use another piece on the other side and make the seam go along the top bump of the shark fin and then between the flat back part and side on the edge, making it nearly invisible. I also recommend using a little adhesive promoter pen to make sure that the edge stays glued down for a long time!
pm if you have questions or want pics.