[Leica] Help: Viso + 2 ext. rings + 200Telyt - exposure?

[Simon Pulman-Jones <spulmanjones@lbs.ac.uk>]

I need to take some close-up pictures and the only equipment that I have 
that can do the job is a Viso, 200 Telyt and two 15mm extension rings.  The 
view through the Viso with this set-up looks great, but before I go ahead 
and start shooting I would be very grateful if any of you could give me 
some advice about exposure compensation and depth of field - I've never 
used this set-up before and a few pointers may help me to limit the amount 
of bracketing that I need to do.

I guess that I should add about a stop for each of the 15mm extension rings 
- - is that right?  And how small should I go with the aperture to ensure 
reasonable depth of field for photographing a camera?  As I stop down I 
find that it is very hard to assess depth of field as the image darkens - I 
am doing a typical ebay set of camera pictures of top and bottom plates, 
front and back, etc., so the maximum depth of field needed would be only 
three inches or so and the minimum acceptable about one inch.

Any advice would be very much appreciated.

Simon,
London.