[Vigilanteman]

Do you have an accountant or someone who does your taxes? Your employee can get nearly a fixed withholding rate by manipulating the W-4 IF the pay scale tends to be more or less even. But if there are big swings in compensation such as payment of bonuses and commissions, it can be nigh impossible.

I believe a signed request from your employee requesting the fixed percentage should be all the CYA evidence you would need. But you'd be wise checking with whomever does your taxes to be sure.

[TheThirdRuffian]

He’s hourly (a drilling rig hand) and pay varies considerably.