All you are trying to do with peak white level is to avoid clipping whenever possible and to avoid eyestrain. If you know when your display clips white (some never clip white at ANY Contrast setting) your only concern is eyestrain. Eyestrain comes from viewing the bright screen against a black background. If you sit so close to the screen you can't see any black around it, you can make the screen as bright as you like without ever having eyestrain issues. The more black there is around the screen within your field of view, the more likely you will be to have eyestrain issues and the dimmer you'll want to set the TV. Sit closer so there's less black area around the TV and you'll decrease the possibility of eyestrain.
Setting the Backlight is actually more important for BLACK LEVEL than it is for 100% white.. maybe. LCD displays aren't particularly impressive in regards to blak level. To get the best black level, you want to find the lowest possible backlight setting where the backlight doesn't mis-behave (gives poor color or odd motion effects). When you find the dimmest setting for Backlight where the display produces good/accurate images, set the Contrast control to achieve the peak white level you need. This method helps you achieve the lowest possible black level. If the TV won't get bright enough with the Contrast control (or if clipping of one or all 3 colors) sets-in when you try to use Contrast to set peak white, your only choice will be to raise the backlight setting, but you still want to try to keep it as low as you can to achieve the peak white level you need.
There is no 1 right number for peak white... "it depends" is the only right answer. The farther you sit from the TV, the more likely you'll need a lower setting (30 fL or less). The closer you sit, the less critical the peak white level becomes. Wen you sit too far away, you can't see all the detail in 1080 images also... and WAY too many people sit WAY too far from their 1080 displays to be able to see all the detail in images. six feet for a 50" panel is just about the sweet-spot, but I'd guess the average distance people sit from 50" panels is closer to 9 feet so they aren't seeing all the detail the picture has to offer.