The design, as it is now, should give you about 1' overhang at head and foot when you lay on the diagonal. That should be plenty, but if you cat cut your edges you may lose some needed coverage. Cat cutters beware.
Please post pics of your finished tarp.
" . . . there's no easy trail to Cache Lake, for it is protected by distance, mile after forgotten mile of woods and water, and it is still clean and clear and safe from civilization." John J Rowlands from Cache Lake Country