Withholding calculations from new W-4P

I recently started a new annuity and was required to submit a W-4P for them to calculate my withholding.  For other annuities I have had I always had 12% deducted and never was required to submit any type of W-4 and usually received a refund at years end on my federal taxes.  I filled out the form as best I could, but when my I received my first annuity payment 31% federal tax had been withheld, way too much, severely impacting my monthly cash flow.  I have contacted IRS phone help and they do not have live support for this topic.  Does anyone know how the various numbers on the W-4P are used to calculate withholding so that I can put in appropriate numbers to get my withholding down to the 12% level?  Thanks in advance for you consideration and help.