I know people here who don't hesitate to put down a few mill when the Power off 9 signals a trade. Personally, I've never used it coz I could never figure out the logic behind it. DeMark isn't the only one who called a top. Lots of "reputed"others did so as well.
Dunno about 20%, but DeMark is a legend not to be taken lightly. I would be very cautious. Read this first and then decide if 20% is possible https://www.elitetrader.com/et/threads/will-pakistan-start-the-next-nuclear-war.384398/
Guess you didn't believe me when I said TD SetUp9 & TD Sequential13, not silly "Power of 9":- https://demark.com/indicators-list/ BTW other using his indi's I have no connection to the man or company.