Two remarks that might help others :

-First task, the last column named „%GT Sum of Attendees" in the pic is the Percent of Grand Total. You can deduce it from the second picture.
You find it in the same menu than Sum, Average, etc, last option = Show value as.

-Further, to avoid a confusion on the Icons conditional formatting : the pic from the exercise "What field should we base this on ?" shows Average of Attendees whereas in my current try, with the right column titles, for it to work, I have to let it on the by default Total Attendees but select Average in Summarization.