Like Peter, I am also on an A70 on 9. Pretty good device actually! I watch videos on it almost every night.
It is easy for me to adjust the brightness to an optimal setting for videos. I simply use any pleasant wallpaper that has a range of tones, and I adjust it to a level that provides good detail in as many tones as possible but also vibrancy. I know it's too high when it starts to bloom/glow and slightly wash out some of the colour. Likewise I know it's too low when it becomes dull. It's very predictable what the optimal setting is. Sand, clouds, skin, etc. are all good things to look at to make sure it's not too much or too little.