You got it! You can use a short piece of anything like a pencil eraser or piece of arrow or nothing. The more you put into the hole where the arrow goes the further back your fletchings go. Or you could do like me an add nothing. You will have to experiment with the tool to see what you like unless you know where you want them already.
The rear edge of my fletchings end up about 1 1/4" from the rear nock. I prefer them there because the fletchings push up against the scope bridge too much for my liking if I go further back. No big deal especially if you don't use Blazers or Fusions which are much stiffer and taller than other fletchings.