I don't quite know what the 21% means either, but the key-like symbols on the right of that head with the 21% underneath is the key to configure the toilet water pressure after filling the tank...need to try around a bit and do it in the order depicted on the key! Since the screen switches away once you got it, no screenshots are possible but trying around with approximation of the 4 levels of each symbol brings you to the solution sooner or later. But the order of them is important, too!