Flash help required, please

[VWGolf]

Help required for Saturday, please. when I'm going to take some important photographs inside a building.

Sometime back, when I was enquiring about Pocket Wizards and suchlike, someone told me that my Nikon D300 can use Commander mode to trigger my SB600 flash gun off the camera. I've not had much flash experience but somehow I got not only the SB600 to fire but also a cheap Jessops flashgun that I bought on eBay. So thank you for that :-)

However, particularly when using a backdrop, although I can reposition/change the power of the flash guns to what I like, I find that the on-camera flash that triggers them, is causing shadows. I have gone through my camera menus but can't work out how to alter the strength of the on-camera flash.

I have gone into the custom settings/bracketing/flash/flash control for built in flash and then not sure what to do next. The options are

TTL

Manual

Repeating flash

Commander mode

I am currently in commander mode with all the options (built-in flash/Group A/Group B) set to TTL

If I change the Commander mode/built in flash to Manual and chose, say 1/16 - will that do it? If I lower the on camera flash, will it still be strong enough to trigger the two flash guns?

Hope someone with some Nikon experience can advise. I've tried doing different things here at home, but becuase I'm in a small room, as opposed to a large Victorian/Gothic hall with wood-panelled walls and painted ceilings, it's a bit hard to tell exactly what difference it makes. Any advice gratefully received.

[Black Pearl]

When in the Commander mode setting turn the in-built flash to 'Off'

Don't worry it will still fire but only at the start and end as a trigger for your SB600