On my Scott I have a slight problem with the indexing but there seem to be 2 adjusters. One is a black plastic nut behind the rear deraillieur and the other is a large barrel adjuster up near the lever. Which one should I use?
mickle is correct in what he writes, but a word of caution; when adjusting either, be careful that you do not go to the full extent of adjustment on the rear adjuster, as it could become detatched from it's housing. Thus making any adjustment redundant!
If your rear mech is set up correctly prior to cable installation there should be no need to turn either cable tension adjuster more than one revolution. The key is in setting the limit screws correctly and then fitting the cable with zero slack. If you are turning the adjuster so much that it unscrews from the mech then something has gone badly wrong with initial setup.