Calculating payroll withholding for small (tiny!) business?

I run a patent law firm, and I occasionally have law students work for me part-time. I don't write enough payroll checks to justify setting up with a payroll service, but I can't just pay my students under the table or have them be 1099-contractors.
Is there a spreadsheet available that will get me adequate withholding numbers so that I can run checks myself? I've fiddled around with Excel for a bit, and I have a spreadsheet that adds up already-paid and expected-to-be-paid wages, calculates the federal tax on that amount (less deductions and allowances), then splits that up into pay-period chunks to give me the numbers for a pay stub. I figure that'll be close, but I don't know if it's close enough for government work. (Perhaps there's enough subtlety in there that the payroll companies have figured out, that make it difficult/impossible to do in a simple spreadsheet? My suspicion is that it's really not all that hard, apart from updating the tables every year.)
Thanks, dhm.
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On Monday, March 11, 2019 at 11:45:18 PM UTC-7, snipped-for-privacy@mersenne.com wrote:

See IRS Publication 15 for the tables, and Publication 15A for allowable "alternative" methods. It sounds as if you're close to one of the alternative methods which must be requested by the employee.
States, even states without income tax, may have other requirements. -- Arthur Rubin, Brea, CA
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