Another way to do this is to set the camera to manual metering and use that to determine the exposure you want for the background. You can then use either the aperture or the shutter speed to control the background exposure. However, there are two considerations (at least) that you want to make: 1) keep the selected shutter speed within the strobe synch speed - that's really not a problem with this photo but it could be with other images that are brighter and you want just "daylight fill flash"... 2) Keep the aperture relatively wide (or open). You don't want a small (say f/8 or f/11) aperture so that that strobe power isn't sufficient or limit the range of your strobe with a small aperture. Again likely not an issue with these image condition, but I would suggest an aperture of something like f/4 or f/2.8.

Now set your SB600 to TTL as you want the strobe to be your main light. The Nikon strobe integration will take over and the light on the subject will be determined by your selection of aperture, strobe power and the camera's automagic adjustment of strobe exposure.

I typically choose an ISO of 400 or 800 for these images on my D700. That gives me reasonable speed for the background exposure, a usable strobe "range" at f/2.8 or f/4 and minimal "grain". There will likely be blue channel noise in the dark portions of the sky, but it rarely shows for modern DSLRs, in prints or typical screen magnifications. If you pixel peep you may find some, but I've only added it as ISO can be chosen for a reason.

Best Regards

Roger
It's still, ISO, aperture and shutter-speed, right?