OK, I think I understand. In my case, I made the rollover in January (and therefore incurred the tax liability in January). I estimate (pretty accurately) that I will owe approximately $45,000 in taxes on that rollover. Because I know the amount I will owe, I can pay $11,250 by each of the 4 quarterly deadlines in 2020 and not have to pay the entire $45,000 by April 15 of this year.
