Rec709 doesn't define a reference luminance or a reference viewing environment. There are some other specs which describe this in various ways which I am less familiar with.
How bright you want depends on the size of the screen, ambient lighting, backlighting, and viewer preference. Most of the recommendations around here are more about pleasing ranges that are commonly targeted, which are rather arbitrary but which fit well within a particular viewing environment.
I mean, just ballpark on something like a plasma, I might target a range of 20-40 foot lamberts depending on lighting conditions and the type of content the viewer is most concerned about (brighter for sports & TV, dimmer for mainly movies for instance). That would be ~70 to ~140 nits, roughly. I would target probably 10 to 20 footlamberts for a large projection screen in a darkish or dark room, again depending.
That's just my viewing preference, I think those are reasonable value ranges, but I'm sure others will chime in as well.
I don't recall the SMPTE specs that describe in more detail the studio environment, but it's important to note that as most of our viewing environments deviate from that type of viewing environment and screen size, we have to make some intelligent derivations based on that standard which lead to different but appropriate conclusions that match how our particular viewing environment differs.