Here is a wonderful chart composed by Mr. Scrivener diagramming the details of the current picture. The letter-counts are based on the text as transcribed on the Codex Sinaiticus Homepage (British Museum), but we have subtracted all marginal notes (which were added independently later), and also the peculiar symbols of Scribe D (the ">" arrow used to fill rows to make the page look fuller.). Still counted as a character was the "dots" indicating verse and paragraph divisions, because these are assumed to have been copied from the original master-copy, and are not the invention of either scribe.
Now lets turn to the chart:
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He has already carefully calculated that the best course for handling Luke's verses will be to squeeze them into SIX of the last columns on the 'cancel-sheet' (folio 77-5 recto last 2 columns, and the 4 columns of the other side, 77-5 verso). This will gain him about a half a column.
Now he calculates the remaining approximately SIX columns of Mark, and to leave the last column nearly empty (but avoiding leaving a whole column), he crams about 60 letters into the last column of folio 77-4 recto. He has actually been too extreme here, and now must write the final FIVE columns of Mark rather sparsely to make sure he spills over a little into the last blank column before Luke.
A glance at the column heights explains why he chose this route. He did not have enough material to stretch Luke to fill an entire SEVENTH column, and even if he had done this by some real spreading, He would have only had just enough space for Mark, filling all the previous columns, without the Long Ending (Mark 16:9-20). This may indeed have been exactly what Scribe A had originally done.
However, Scribe D, likely the Overseer and Corrector of the scriptorium, knew this would be highly unsatisfactory, and wished to at least leave a nearly blank column to tip off future users and enable them to copy in the Ending if they chose to. Scribe D then, was aware of the Long Ending. Even though he did not allow sufficient space for it, due to his desire to keep the look of the manuscript professional and standardize the book-seams, he did feel compelled to make sure that at least the option was available and the problem highlighted.
In this strategy, Scribe D mimicks almost exactly the behavior of the scribe of Codex Vaticanus, who also leaves just enough space to allow for Mark's Ending, but in that case he also left a rather awkward completely blank column between Mark and Luke. It seems clear that our Scribe D had seen that solution before and wanted to improve it by extending Mark into the blank column, and give the appearance of a normal manuscript.
We only need try to reconstruct what the original Scribe A had done on the pages now lost, which ought to explain how the 'need' for a substitution arose.