I guess a professional discussion is not your style, nor is the allowance for a different perspective.
I was trying to share what payroll pro's do (not give any tax advice to employees, including help with preparing a W4). I was thinking those who tell their clients to withhold a flat percentage are not up to date with the W4 not having a flat percentage option might want to give useful advice, rather than giving advice (while accurate) which cannot be used, and could actually take more of the preparer's time to have to calculate a suggested W4 item after already calculating a flat percent. The client who has received a flat percentage rate to use is goign to complain that someone is a problem, maybe they will blame their employer, maybe they will blame their preparer. Why take that chance when it is easy to do the figuring and give them a W4 ready for their signature?
The new W4, if it is similar to what was already proposed, will make it easier for the preparer to give useful numbers, since it allows for dollar entries, not made up status and allowance. The numbers should already be at the preparer's fingers. The complicated part will be when a taxpayer prefers not to divulge (to their employer) their additional revenue amounts, but the form allows for entry of credit/deduction amounts instead of (maybe also in addition to, I cannot remember) specific income amounts. Whether or not we like it in principle (it really should not be as hard as it is), complication in tax processing is what we all make a living on... candidly, no only likes complication, but a real flat tax method would have us all looking for other work - which is timely given the proposal to prevent the government from providing free efile methods in perpetuity I was just reading about.