I've adjusted the cable on my Summit lots' of times.

Try is down low to see how it works but it's pretty easy. For one, on the top you can just put the strap you secure it with one adjust all you want.

For the bottom, I bring it up close to the top like you are climbing and then set it in the tree.
Put your knees in the seat, facing the tree, and pull up the end away from the tree to catch them with your toes and push it against the tree. You will still have both hands free and it's easy to hold it there as long as you need. The cable will be loose and you can do what every you want to with it.
It takes about 4 times as long to type it out as it does to just do it..
Try it. You'll see, it's easy.