I have used about 35 to 37 foot for SRT. I kept throwing a throwball over a nice crotch and realized my rope was not long enough so I went to 50 feet of rope. Now I can hunt hunt from the tree I want.I see most people recommend carrying 40' of rope to rappel down the tree. If you use 6' of rope to go around the tree with your knot for the link and your 18 foot up in the tree you could get away with 30' comfortably.