A useful trick you can use is that as long as it has been 6 months since giving your chancellor the fabricate claims mission, he can be moved while the successful claim popup is up in order to change the location of the fabricated claim. So move him around after every county claim (to maximize his chance for county claims), and then if he succeeds in a duchy claim, move him to the target duchy you really want before accepting the successful claim popup. Then you will earn your duchy claim on your true target. This is far superior to just letting him sit in one place for years and years hoping for a duchy claim (and missing out on all the successful county claims).
*Edit. Also, if it *hasn't* been 6 months since the mission was assigned and you still need to relocate him, just hire someone else and appoint them to the location instead. The claim will still work regardless of his skill, because once the event is popped up the outcome is set. The only thing that happens when you click to pay the cost is that it reads the location of your chancellor. Who he is, or where he was when you succeeded doesn't matter.