Since I'm not really familiar with the way the D70 and sb600 work together
in auto mode, I'll give you some general guidelines and techniques, and
you work it out from there.

First, NEVER let your camera's shutter speed go above the flash sync
speed.  It can be less, but never more.

Second, since the flash is just "filling," I make the flash, by whatever
means possible, output at 1.5 to 2 stops LESS than what the natural light
exposure would be.  So, for example, in the old days of film cameras and
auto-flash, if I were shooting 25 speed film, I would set the flash for
100 speed and it would automatically output 2 stops less light. With some
digital cameras, you can set via menu for the flash to output less. I use
this technique with a little Olympus C3030 I use for snapshots. Works
quite well.

Third, with backlit or subjects in open shade, I find that having the
flash supply 50% of the light for the exposure gives a nice look.  That is,
you would underexpose the ambient/natural light by 1 stop, and set the
flash to supply the other 1 stop of light.

Fourth, ABSOLUTELY do tests before the actual shoot to work out the bugs.

Stefan

For the most part in photography, what you see is close to what you get.
I've always been amazed by the insistence of people to stand out in the
bright sun to take photos, even to point of proudly telling me that they
went out of their way to schedule the wedding, especially the photo time,
when the nicely landscaped backyard is in full glorious sun.

For the most part, when you move the subjects out of the direct sun you
remove a bunch of problems and often create a very nice portrait.   That
space between the edge of the light and the deeper shade, that transitional
phase, the penumbra, is your sweet light.  Look at your subject,  have them
step back slowly from bright sun on their head, to just where the direct sun
disappears, step back a couple more inches, now the skin glows from within,
the eyes sparkle, the hair seems alive.  This my friend is sweet light.
Take another step back and you lose it all, the light goes from soft and
diffuse to mushy and the subjects merge with the muddy shadowed background.

Some of the best headshot, glamour light, all those make up ads of
astonishingly pretty faces in those fashion magazine thick as phonebooks,
can replicated with something as simple as opening a door and having your
model step back from the bright light, the space is between the edge of the
full light and shadow, *just* as the light starts to fall off in volume.

so save the fill flash for when you must shoot in out in the direct sun.
Fill flash is usually one stop under the key light or sun light exposure.
Assuming you are shooting at ISO 100 or 125, your normal BSD or bright sunny
day exposure is 125th @ f/16   so your flash should be set for f/11.

All the time I see photographers, usually wedding photogs, (one of my other
jobs is a wedding officiant)  look right at a beautiful situation and then
wash it out with a flash, THEN... they take the groups outside and they turn
the damn thing off.