40 is not max charisma. You get 10 from perfume, 10 from self help book, 20 from shower and 30 from skills (Serotonin), so max is 70. Anyway, the chance to pass the check is equal to charisma in %. So if you have 40, you have 40% chance to pass the check. This is determined each time you start to dance. So even if you did not pass with a girl first time, you have another 40% chance if you start to dance again. If your charisma is 70, it is 70%.At the nightclub charisma check fails , even with the max 40 charisma.