What Matte said is the best advice you will ever get. As a broadcast engineer, I have to line up content for two stations. We check the bars and tone. Then find what appears to be the brightest and darkest part of say a commercial, then we raise the blacks so they don’t go under 0 IRE, and lower the whites so they don’t go above 100 IRE. Audio is set with a combination of a dialnorm meter and a VU/ppm meter.
The volume of content that is ingested in a day prevents us from being able to ride the levels through-out, so we adjust for the maximums. In a perfect world there should be no adjustment after setting to bars and tone. But I have seen some wacky ass levels from the highest end of commercial productions houses.
But hey, that is where “Never Twice the Same Colour” came from. Everyone has a different generator for bars and tone.