You can use the Equaliser (Pre Amp slider) or Parametric Equalizer in DSP to adjust the volume as you wish for any Zone. Adjustment is by decibels rather than %, but I'm sure you will be able to find a setting. Probably best to set Internal Volume at 80% to match Video requirement, and adjust Audio volume down, rather than setting Audio and boosting Video.
But Volume Leveling based on Audio and Video analysis should set the Internal Volume correctly for you anyway, unless the difference is just a preference thing. I would understand that. Even with boosted frequencies for dialogue, I often turn the volume up a little on some programs so that I can understand the quiet dialogue.
Do you have Volume Leveling turned on?