[Leica] Fill flash help

[Kip Babington <cbabing3@swbell.net>]

Can someone either give me some help, or point me to a quick & dirty
exposition on line, on how to add just a bit of fill flash to a Leica M
(on topic) portrait setup I have to do this week?  I've never had to do
this before, and am having a brain spasm trying to think through what to
set where on camera and flash.

The physical setup is a group shot of four vocalists (including my wife
- - this has to work) after a performance in a church hall.  I did a roll
there last week with just available light, but while there's enough
light to get a picture (on HP5 @ 800) the light is all from directly
overhead and the eye sockets and foreheads have unflattering shadows.
The situation permits a tripod (which I had to use last week too -
exposures were about 1/60 @ 2.8) but won't allow me to set up my
umbrella stands to get a lot of light with balance.  (I don't want to
take just a straight flash shot.)   So I'm thinking I can just lighten
up the facial shadows with a bit of fill from a shoe mount flash held
about arm's length  away from the camera (I can dial the flash output
down to 1/16, and add a wide angle diffuser to further reduce power, if
need be.)

I started to plan on using my N**** with whiz-bang TTL everything
dedicated flash, but after reading the flash instruction book I can't
figure out how to adjust the ratio of available light to flash (it seems
to be a "use these settings and we'll do fill flash for 'ya" sort of
deal) and since I'm not sure what I'll get automatically I'm thinking
I'll just do it myself with a real camera (real photographers don't use
auto . . .).

Anyway, I have ambient and flash meters, both incident and spot (but no
instruction books, which might answer this question) to help me get a
basic exposure - I used 'em last week, along with the M6 meter, and got
decent "average" negatives.  But like I say, I need to add a bit to the
faces and for the life of me I can't figure out how to use the meters to
tell me what to set on the camera and flash. I'll bracket a couple of
stops (with the flash power control, I expect) either side of my
starting point, but can someone give me a clue how to find the starting
point?  Off list is probably better if there's any back and forth
necessary, but if it's just a two or three step process there may be
others on the list who could use the info too.

Thanks in advance.

Cheers,
Kip