Although the dates are at the extreme end of the acceptable range (just before 2000 but well after 1980), this is a good match for the short story "The Great Escape", written by Ian Watson. He published it in "Dante's Disciples" in 1996. Later he republished it as the title story of the collection "The Great Escape", 2002.
The story is told from the view point of the Impresario Angel, an angle whose job is to survey hell using a lens which projects images to heaven. At the peak of heaven is the Quint (the Quintessence) which is surrounded by many layers of angels. The Impresario Angel is very remote from the Quint.
Generally it avoids conversing with the demons or the damned souls. But it becomes curious as what demons do when no angel is watching them. It discovers that the demons have harnessed mortals to the Ark (described as a prison hulk and slave ship, used for punishing the damned souls), and are rowing it towards the Quint. It is a long way off.
The Ark is rooted in hell, so they are dragging hell with them. Thus they believe that the guards to keep them in hell won't stop them, since they are not leaving hell. The Angel cannot leave anymore - hell has shifted enough that its portal is no longer accessible. It is "appalled and elated" to go with them, appalled since they intend to attack the Quint, elated since otherwise it will never see the Quint.
Some quotes:
Ours would be a long journey - of two hundred and fifty-five point nine six quintillion miles.
The following exchange sounds like the exchange dimly recalled in the question.
Our demon informant taunted me: "By now the distance to the Quint is only one hundred and seventy nine point nine parasangs!"
The demon who lay under the theodolite corrected him. "Point nine nine."
"Progress is being made!" snarled our demon.