What level should flats be

[blinky]

I am trying to get the correct exposure for my QHY8 flats. I have a monitor that I lie flat on top of the scope with the brightness turned right down. I have taken 10s exposures and I thought the level was OK but on stacking in DSS I can see some concentric rings on the stacked image. So...... does this look OK or should I lower the exposure?

[blinky]

Here is the stacked image with a stretch done in Photoshop