I still have a slide rule. Most of you may never have seen one. I don't
know where you might buy one, and I don't know who might teach you to
use one, but at least when I tell you I'm 76 years old, you'll believe
me.

If you are contemplating working in RS232 I suggest that you get the book
"Serial Port Complete" by Jan Axelson.  Jan has wrote a number of books in
this "vane" including "parallel port complete" "usb port complete".  she is
a good, thorough, and understandable writer.

It is the enlargment ratio that counts, some enlargers indicate it directly,
if yours don't, you can measure the same (and *complete*) side of the
projected image.
The exposure is proportional to the projected surface, this is an
approximation but normally sufficient for photographic purposes.

For the filters adjustment factors:

B/W Multigrade
- classic MG acetate filters have usually constant filter factors, except
for the harder grades needing usually a full stop more exposure.
  This is done to allow common grades requiring less filtering to be
faster.
- using a color head for MG printing requires some more calibration work.
  There are mainly two techniques:
  - single filtering (only one filter used at a time) and use a correction
factor for each grade
  - double filtering (Yellow + Magenta filters used together) to maintain a
constant factor. Commercial acetate filters are in fact of this kind: a
  "premix" of Yellow and Magenta (perhaps some grey) having a constant
exposure factor.
  Double filtering may, like for acetate filters, use a second correction
factor for harder grades for the same reason.

Two good descriptions of the double-filtering method:
- Paul Butzi: http://www.butzi.net/articles/vcce.htm
- Ralph W. Lambrecht/Chris Woodhouse:
http://perso.wanadoo.fr/apple2/CntrstCtrlClrEnlargers.pdf (extract of their
book "Way Beyond Monochrome" ISBN: 0863433545)

I do not use double-filtering any more as my computer program stores and
uses the filters factors directly for exposure calculations.
To calibrate your equipment and paper, a transparent step wedge (Stouffer,
Kodak, ...) is strongly advised.
You may find that you have strange results when doing such calibration work,
I had it until I understood they were originated by voltage fluctuations,
but this is another story ...

Color work:
This may result in a huge amount of tests to find all combination of filters
factors ...
I'm lucky that my enlarger came with a chart giving these correction
factors, I just had to retype them in a text file used by my program, I had
only to add an algorythm to find the "in-between" filters factors and voil?.

Regards,