Andrew, Pierre: After some testing, I checked into GIT Andrew's waveform mod to add the additional IRE 92.2% + 6.3% lines for the waveform. Interesting discussion - past and present.
I had forgotten all this complexity and the multiple distinctions
between the IRE values that only concern analog video and the Waveform monitor (VideoScope) of Cinelerra-GG which only calculates % of the digital 0-255 scale.
It's not easy to understand that a YCbCr source image (16-235) corresponds to 6.3-92% on the Waveform grid, especially since there are no visual cues on the grid to indicate this and to show the video level overruns that could cause problems depending on the destination of the editing ...
I added (by trial and error!) those two lines ... they assume WAVEFORM_DIVISIONS 12
but I guess they only valid for some types of footage and some setting of mpeg/jpeg toggle ....