There's not really a standard. You can check a bunch of real-world examples here:
https://forums.azbilliards.com/threads/table-difficulty-factor-tdf-for-measuring-table-toughness.324408/
Earl was pretty nervous at that Mosconi. He made a few very un-Earl-like plays. I remember Shane having to help him move his ball-in-hand when Earl couldn't reach it.