Comment 9 for bug 548918

Although the HP-7475A manual does state that "[t]here are approximately 40.2 plotter units per millimeter", the standard is that one HP-GL unit = 1/40 mm. Physical device units are always an approximation depending on temperature, humidity, and plotting media. If you want accurate output, you'll always need to measure and calibrate.

I have a couple of other HP manuals here. Here's what they say:

The HP-GL/2 and HP RTL Reference Guide: A Handbook for Program Developers, page 12 (Hewlett Packard, 1996. ISBN 0–201–63325–6):

    Plotter-units Equivalent Value
    1 plotter-unit = 0.025 mm (≈ 0.00098 inch)
    40 plotter-units = 1 mm
    1016 plotter-units = 1 inch
    3.39 plotter-units = 1 dot at 300 dots per inch

(available at http://www.hpmuseum.net/document.php?catfile=213 )

HP-GL Programmer's Reference Manual, page 3 (Hewlett Packard, 1984):

    The plotter unit is the smallest possible addressable move
    and is 0.025 mm (approximately 0.001 in) in length.

HP-7470A Graphics Plotter - Interfacing and Programming Manual page 2-3 (Hewlett Packard, 1982, 1984):

    The plotting area is divided into plotter units; one plotter unit equals
    0.025 mm. There are approximately 40 plotter units per millimetre, or
    approximately 1000 plotter units per inch.

(available here: http://scruss.com/wordpress/wp-content/uploads/2014/03/HP-7470A_Graphics_Plotter-Interfacing_and_Programming_Manual.pdf)

I'd say that the 40.2 number is an anomaly in the HP-7475A manual, and that the 40 units per mm standard should be used.