Printing puzzle

[David Epstein]

Jacqueline Gay lucidly explained printing settings, including this:
      Every printer has a hard, physical limitation on how far from  
the edges
      it will print. If you get a half-inch unprintable margin at  
100% scale,
      it will still be the same at any other scale.

Yes, the unprintable margins are fixed, but what are they?   
PrintRectangle reports their size in screen pixels, but that will  
only be their size on the printed page if the page setup scale is 100%.

It appears that LC lets us either
   (a) use the full printable area of the paper (print card into rect  
the printRectangle), OR
   (b) place things at precise locations on the paper (leave a safe  
margin of empty space on all sides of a card whose dimensions are  
equal or proportional to the printPaperSize, then print into rect  
“0,0,” & the printPaperSize)
  – but not both.
Unless my script can know what scale is governing the result of the  
printRectangle, it cannot position an object that will be exactly one  
inch from the top of the printed page using method (a).

If the left and right unprintable margins were always symmetrical,  
the "pageSetupScale" I'm looking for could be derived as
100 * item 1 of the printPaperSize / ( item 1 of the printRectangle +  
item 3 of the printRectangle).
But this symmetry was found on only one of the two printers I tested.

David Epstein