Not sure where you are seeing those choices. If you choose to have tax withheld from your SS you chose a percentage, and in most cases that is also what you would do for the amount to be withheld from a pension.
IF YOU WANT TO HAVE TAX WITHHELD FROM YOUR SOCIAL SECURITY BENEFITS
https://www.ssa.gov/manage-benefits/request-withhold-taxes
https://www.irs.gov/forms-pubs/about-form-w-4-v
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