I have a HF hood, I also wear tinted safety glasses under the hood. Together they are probably equal to an 11 filter rating and can go darker.
Why do I use the Safety glasses? Well you should never trust just the hood, safety glasses help keep sparks and other things out of your eyes that a hood alone cant do. Second is they will help you not get flashed from behind the hood or if you don't get it down in time.
My eyes would get tired after about an hour, after I started wearing the tinted Safety glasses I could weld for hours and my eyes were fine.