[Groff] Converting basic units to inches in PostScript

[John Gardner]

I'm using the dl register to set page indent with .in, which works fine in
nroff, but generates absurdly large sizes in groff. For instance, a
diversion with a width of 624 in TTY output becomes 286946 in PostScript.

I'm aware this is connected to the matter of device resolution, but is
there any way to reliably convert the device's "native units" (?) into
inches, or what-have-you? I tried making use of the .H register, but it's
reporting a value of 1 for PostScript. Oddly, it's reporting 24 for
terminal output.