The current forms are better than I can remember, as there are options to effectively set a flat dollar amount (which can be a % calculation).
It is just silly and to tell them something they cannot legally enter, a flat %. While someone's "effective" tax liability can be calculated as a % for a particular year, there is no such withholding method available, so please stop telling someone their WH needs to be a flat %. A real pet peeve of mine, sicne such statements mean I, after the employer gets to deal with the employee's demand, has to tell the employer and employee the law does not permit such a selection.