🚀 go-pugleaf

I recently came across this problem and wonder if anybody can offer a
helping hand on this:

I tried to print a perfect square on an Apple LaserWriter II NT using

	%!
 	/mm {72 mul 25.4 div} def
	0 setlinewidth
 	newpath
	40 mm 40 mm moveto
	40 mm 90 mm lineto
 	90 mm 90 mm lineto
	90 mm 40 mm lineto
	closepath
	stroke
	showpage	

But the square that I got was distorted. It is approximately 0.5 mm longer
in the y direction (11" direction) and 0.5 mm shorter in the x direction
(8.5" direction). It is essential for us to get an undistorted printout.

Does anybody know what is the reason for this? Is it because of the conversion
factor that I use in defining mm (doesn't make sense to me since I should
get the same error in both direction, not one longer and one shorter)? Is
there something that I should define in the above that I missed? Is it
because of the aspect ratio (does a laser printer have aspect ratio?) of the 
laser printer? If it was in fact due to the laser printer itself, is there
a way to compsensate for the distortion other than specifying something
like 'x y scale' (it is not easy to measure the exact lengths to find
the scaling factors) in the code?

Thanks in advance.

Andy