Generally, I like using achievements to figure out where people called it quits on a game. Like Saints Row the Third. 90% of people cleared the first mission, but the percentages drop with each successive story achievement until you've got the achievement for the last mission which only 27.9% of players bothered to finish. Or you have Hades where around 50% of players just never finished a run of the game and only 25.6% completed enough runs to see the main ending.