Postscript: Rescaling and resizing page at the same time - resize

Question

For a plotting project of mine I was following the How-to by Eric Weeks to rescale my post-script page so that one unit matches 1 cm of length. The header of my PS file looks like this:
%!PS
matrix currentmatrix /originmat exch def
/umatrix {originmat matrix concatmatrix setmatrix} def
[28.3465 0 0 28.3465 0 0] umatrix
It does the job but the other thing I need is to resize the page from US Letter to A4. According to, e.g., Postscript - document size I should setpagedevice with something like:
<< /PageSize [595 842] >> setpagedevice
However I cannot make it work. When I put it in front of the matrix redefinition, it takes no effect. When I put it after the matrix redefinition (even translating the new size to cm) it resets the matrix scale and the page still comes out as US Letter, only the drawing is scaled down because the coordinates are now in pts.
How can I both rescale the page and define its size?
Edit: I am attaching a MWE presenting my problem. This draws a rectangle that would plot a box around the page, leaving a 10-mm margin if the page was A4-sized. On a letter-sized medium it shows how the paper is shorter but wider.
%!PS
matrix currentmatrix /originmat exch def
/umatrix {originmat matrix concatmatrix setmatrix} def
[28.3465 0 0 28.3465 0 0] umatrix
0.020000 setlinewidth
1.0 1.0 moveto 1.0 27.7 lineto 20.0 27.7 lineto 20 1.0 lineto 1.0 1.0 lineto
stroke

Answer 1

Is there another call of setpagedevice in the file which sets the letter format? The call executed last will win for the page size, and setting up the matrix differently has do be done after that call since it resets all graphic settings when setting up the page device.
If you cannot find where it is being called, you could redefine the setpagedevice operator in userdict. That might work or not, depending on how the whole PostScript file is constructed. In the redefinition you would create a new dict operand to setpagedevice, copy all entries but replace any PageSize by your desired value.
This might do:
userdict /setpagedevice {
dup length dict begin
{
1 index /PageSize eq {
pop [595 842]
} if
def
} forall
currentdict
end
setpagedevice
} bind put