I was doing a graphics for my new game. I spent a few hours creating and finally I had a good result on the real device.
oops my device was with 15% of bright. I set it to 50% and the result was very bad.
So it seems that i´ts better leave the device at 50% because many said they use that way.
100% but I'll set it to ~70% before sleeping just so it's easier on the eyes when first waking up. As far as I know, all of my customers' tablets are set to 100%, as well. Most are used outside or in a well-lit environment.