Basically, you wanna sight it in at 20y and put a mark down. Then back up to the furthest distance you can accurately shoot and slide the pin down until the pin is dialed in again. Mark this spot, then find the sight tape that matches up with the distance you shot (same gap between 20 and X yards).
You can get replacement tapes from HHA, or one of us with OnTarget can help. I have it, as well as Milo and Jagger.