The hammock supports/ridgeline don't form a straight line, especially when you climb into the hammock. Unless you want your tarp up high (not bad when you only expect light showers with little/no wind) you need to tie it below where the huggers will meet the tree. The tarp goes more-or-less straight across and the (comfortable) hammock sags below it. Note you should practice putting the tarp up, then the hammock under it. Just imagine it's pouring rain, and when it really is you'll be glad you did.