% followed by a number between 1 and 99 is considered a placeholder for QString::arg(). Here you are not using arg() so feel free to ignore the warning
@VRonin
I don't see this documented in QString::arg() or tr(), but is this a case where a double % would mean a literal %? So if the user wants to get rid of the warning:QObject::tr("... after 100%% usage")?