It will vary a lot according to the brightness of your room and what you're watching. I'd just try a few different settings until you get the best highlight detail without any hint of greyness in solid blacks.
Or you could just use the Black Corrector and Advanced Contrast Enhancer - in Advanced Settings when the Picture Mode is set to Custom - and see whether that delivers the results you require. See p24 of your manual.
Thanks Andrew, I figured it would depend on the ambient lighting conditions. So presumably this is not really a fixed setting as it will vary from a dark room watching a movie to a bright room watching daytime telly.
I might try the automatic setting for the light sensor and see how that works.
Seems to make sense to me. I think it works on picture brightness, not ambient brightness AFAICR, but it's a good place to start, anyway. You should soon find a setting that'll be a workable compromise for different programme material and ambient lighting conditions