|I have been writing the factory OSCCAL value read from windowed 12C509/JW's
|onto the bottom of the chips.  I have been using the Speedball "Painters"
|Metalic Marker to do this.  It is a fine-point pen that writes in paint
|rather than ink.  I use the gold color (#7328).  I find them in craft and
|stationery stores for ~$3US.

What about having a programmer automatically determine the best
value to use whenever an EPROM device is programmed.  It would
only cost 7 words of code space from $1F8 to $1FE or $3F8 to
$3FE to burn the test program, and it would then eliminate the
need to worry about the calibration constant.  I've posted this
idea before but not had time to implement it.  Has anyone else
tried it?