Most of our income is Roth conversions, but I do have a small pension. What I do is calculate our taxes for the year, divided that number by 12 and then set my pension federal withholding taxes to be equal to that amount.
I normally do about 90% of my Roth conversion in January and then top it up in December... so the taxes on the Roth conversion are paid in arrears because I hve them paid as withholding from pension. If I did estimated payments I would need to pay the federal tax on my Roth conversion in April rather than over the year.