Split *before* page numbers

[tigregalis]

Something I typically do is scan several documents in one go and split files electronically into individual documents.
What I typically do is scroll through the scanned file and identify which are the first pages of each document.

An illustration below, column A shows the page numbers for the first page of each document. Column B shows what I have to enter into PDFsam (1 less).

What I have to enter into PDFsam (as there is only an "after"):

This workflow would be nicer if I could simply define which numbers to split before (i.e. split before the first page of each document).