Totally off the wall W4 idea (United States tax)?

Each year, when I file, I can see what percentage of my income goes to taxes. It's usually 6 and change due to a lot of deductions and the child care credit. Wouldn't it make a lot of sense to allow people to just pick a percentage of their pay to have withheld based upon their previous year's tax liability? So, all I would do on a W4 would be to put down a simple number, 7% to have withheld. Does this make sense?