But, Excel determines before you've selected a printer where the break between columns will be ... that is, which columns fit on the current page.

Doesn't that mean that considering printer pixels is not the issue? It appears that you should be able to compute the sum of the column widths (as reported by Excel), divide this by some constant which is the dependent on default font in Excel, and that should tell you how many inches wide the printed output will be. (Even if this is true, there doesn't seem to be a method yet to determine what that constant would be.)